Light emitting device and electronic appliance using the same

ABSTRACT

A light emitting device comprises a pair of electrodes and a mixed layer provided between the pair of electrodes. The mixed layer contains an organic compound which contains no nitrogen atoms, i.e., an organic compound which dose not have an arylamine skeleton, and a metal oxide. As the organic compound, an aromatic hydrocarbon having an anthracene skeleton is preferably used. As such an aromatic hydrocarbon, t-BuDNA, DPAnth, DPPA, DNA, DMNA, t-BuDBA, and the like are listed. As the metal oxide, molybdenum oxide, vanadium oxide, ruthenium oxide, rhenium oxide, and the like are preferably used. Further, the mixed layer preferably shows absorbance per 1 μm of 1 or less or does not show a distinct absorption peak in a spectrum of 450 to 650 nm when an absorption spectrum is measured.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates a light emitting element having a layercontaining a light emitting substance formed between a pair ofelectrodes, a light emitting device including the light emittingelement, and an electronic appliance.

2. Description of the Related Art

A light emitting element, which has recently been attracting attentionas a pixel of a display device or a light source of a lighting device,has a light emitting layer between a pair of electrodes. When currentflows between the pair of electrodes, a light emitting substancecontained in the light emitting layer emits light.

In development of such a light emitting element, one of importantproblems is to prevent short-circuiting caused between a pair ofelectrodes of the light emitting element. One reason of shirt-circuitingcaused between a pair of electrodes is a projection generated on eachsurface of the electrodes. Such a projection is generated when ITO andthe like are crystallized, for example. The short-circuiting between thepair of electrodes can be suppressed by reducing roughness of eachsurface of the electrodes by covering the projection with a thick layer.However, by providing the thick layer, driving voltage of the lightemitting element is sometimes increased. Therefore, a technique by whicha thick layer is provided without increasing driving voltage has beendeveloped. For example, patent document 1 discloses a technique by whicha mixed film in which divanadium pentoxide and α-NPD (note that, α-NPDis also referred to as NPB) are mixed is provided. It is suggested thatthe short-circuiting can be prevented by providing such a mixed film inthe patent document 1.

It is thought that the technique as disclosed in the patent document 1is extremely effective to reduce short-circuiting caused between a pairof electrodes. However, with respect to absorption spectrumcharacteristics of the mixed film, in which divanadium pentoxide andα-NPD are mixed, there are large variations in absorption intensitybeing dependent on an absorption wavelength especially in a visiblelight region. Accordingly, the amount of light absorbed in the mixedfilm is varied depending on wavelengths of light emission, therebycausing difference in light extraction efficiency for each wavelength oflight emission.

-   Patent Document 1: Japanese Patent Application Laid-Open No.    2005-123095

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a light emittingelement, which can reduce an operational defect due to crystallizationof a compound contained in a layer provided between a pair ofelectrodes, and a light emitting device and an electronic applianceusing the light emitting element. Moreover, it is another object of thepresent invention to provide a light emitting element, which can preventvariations in light extraction efficiency being dependent on colors oflight emission, and a light emitting device and an electronic applianceusing the light emitting element.

One feature of a light emitting element of the present invention is thata mixed layer containing an organic compound such as an aromatichydrocarbon which does not contain a nitrogen atom, more specifically,an organic compound which dose not have an arylamine skeleton, and ametal oxide is provided between a pair of electrodes. Further, whenmeasuring an absorption spectrum, the mixed layer preferably showsabsorbance per 1 μm of 1 or less, or does not show a distinct absorptionpeak in a range of wavelengths of 450 to 650 nm.

In an aspect of the present invention, a light emitting element includesa layer containing an aromatic hydrocarbon and a metal oxide between apair of electrodes. The aromatic hydrocarbon is not particularlylimited; however, an aromatic hydrocarbon having hole mobility of 1×10⁻⁶cm²/Vs or more (and more preferably, 1×10⁻⁶ to 1×10⁰ cm²/Vs) ispreferably used. As such an aromatic hydrocarbon, for example,2-tert-butyl-9,10-di(2-naphthyl)anthracene; anthracene;9,10-diphenylanthracene; tetracene; rubrene; perylene;2,5,8,11-tetra(tert-butyl)perylene; and the like can be given. As themetal oxide, a metal oxide showing an electron accepting property to anaromatic hydrocarbon is preferable. As such a metal oxide, for example,molybdenum oxide, vanadium oxide, ruthenium oxide, rhenium oxide, andthe like can be given. The aromatic hydrocarbon and the metal oxide arepreferably mixed in the layer so that when the layer containing thesesubstances is measured by an electron spin resonance technique, thelayer shows a peak derived from resonance of unpaired electrons, andtransmittance in a spectrum of 450 to 650 nm is 80% or more, andconcretely, 80 to 95%, or absorbance per 1 μm in the spectrum of 450 to650 nm is 1 or less, and concretely, 0.3 to 0.8.

In another aspect of the present invention, a light emitting elementincludes a light emitting layer between a first electrode and a secondelectrode, and a layer containing an aromatic hydrocarbon and a metaloxide between the light emitting layer and the first electrode. Whenvoltage is applied to each of the first and second electrodes so thatpotential of the first electrode is higher than that of the secondelectrode, a light emitting substance contained in the light emittinglayer emits light. The aromatic hydrocarbon is not particularly limited;however, an aromatic hydrocarbon having hole mobility of 1×10⁻⁶ cm²/Vsor more (more preferably, 1×10⁻⁶ to 1×10⁰ cm²/Vs) is preferably used. Assuch an aromatic hydrocarbon, for example,2-tert-butyl-9,10-di(2-naphthyl)anthracene; anthracene;9,10-diphenylanthracene; tetracene; rubrene; perylene;2,5,8,11-tetra(tert-butyl)perylene; and the like can be given. As themetal oxide, a metal oxide showing an electron accepting property to anaromatic hydrocarbon is preferable. As such a metal oxide, for example,molybdenum oxide, vanadium oxide, ruthenium oxide, rhenium oxide, andthe like can be given. The aromatic hydrocarbon and the metal oxide arepreferably mixed in the layer so that when the layer containing thesesubstances is measured by an electron spin resonance technique, thelayer shows a peak derived from resonance of unpaired electrons, andtransmittance in a spectrum of 450 to 650 nm is 80% or more, andconcretely, 80 to 95%, or absorbance per 1 μm in the spectrum of 450 to650 nm is 1 or less, and concretely, 0.3 to 0.8.

In another aspect of the present invention, a light emitting elementincludes a light emitting layer, a first mixed layer, and a second mixedlayer between a first electrode and a second electrode, and when voltageis applied to each of the first and second electrodes so that potentialof the first electrode is higher than that of the second electrode, alight emitting substrate contained in the light emitting layer emitslight. In such a light emitting element, the light emitting layer isprovided to be closer to the first electrode than the first mixed layer.The second mixed layer is provided to be closer to the second electrodethan the first mixed layer. The first mixed layer contains a substanceof one or more of an alkali metal, an alkali earth metal, an alkalimetal oxide, an alkali earth metal oxide, an alkali metal fluoride, oran alkali earth metal fluoride; and a substance having an electrontransporting property. As the alkali metal, lithium (Li), sodium (Na),potassium (K), and the like can be given here, for example. As thealkali earth metal, magnesium (Mg), calcium (Ca), and the like can begiven, for example. As the alkali metal oxide, lithium oxide (Li₂O),sodium oxide (Na₂O), potassium oxide (K₂O), and the like can be given.As the alkali earth metal oxide, magnesium oxide (MgO), calcium oxide(CaO), and the like can be given. As the alkali metal fluoride, lithiumfluoride (LiF), cesium fluoride (CsF), and the like can be given. Asalkali earth metal fluoride, magnesium fluoride (MgF₂), calcium fluoride(CaF₂), and the like can be given. Further, the second mixed layercontains an aromatic hydrocarbon and a metal oxide. The aromatichydrocarbon is not particularly limited here; however, an aromatichydrocarbon having hole mobility of 1×10⁻⁶ cm²/Vs or more is preferablyused. As such an aromatic hydrocarbon, for example,2-tert-butyl-9,10-di(2-naphthyl)anthracene; anthracene;9,10-diphenylanthracene; tetracene; rubrene; perylene;2,5,8,11-tetra(tert-butyl)perylene; and the like can be given. As themetal oxide, a metal oxide showing an electron accepting property to anaromatic hydrocarbon is preferable. As such a metal oxide, for example,molybdenum oxide, vanadium oxide, ruthenium oxide, rhenium oxide, andthe like can be given. The aromatic hydrocarbon and the metal oxide arepreferably mixed in the layer so that when the layer containing thesesubstances is measured by an electron spin resonance technique, thelayer shows a peak derived from resonance of unpaired electrons, andtransmittance in a spectrum of 450 to 650 nm is 80% or more, andconcretely, 80 to 95%, or absorbance per 1 μm in the spectrum of 450 to650 nm is 1 or less, and concretely, 0.3 to 0.8.

In another aspect of the present invention, a light emitting elementincludes n (n is a given natural number) pieces of light emitting layersbetween a first electrode and a second electrode; and a first mixedlayer and a second mixed layer between an m-th light emitting layer (mis a given natural number of 1≦m≦n) and an (m+1)-th light emittinglayer, wherein when voltage is applied to each of the first and secondelectrodes so that potential of the first electrode is higher than thatof the second electrode, a light emitting substance contained in thelight emitting layer emits light. The first mixed layer is provided tobe closer to the first electrode than the second mixed layer. The firstmixed layer contains a substance selected from the group consisting ofan alkali metal, an alkali earth metal, an alkali metal oxide, an alkaliearth metal oxide, an alkali metal fluoride, and an alkali earth metalfluoride; and a substance having an electron transporting property. Asthe alkali metal, lithium (Li), sodium (Na), potassium (K), and the likecan be given here, for example. As the alkali earth metal, magnesium(Mg), calcium (Ca), and the like can be given, for example. As thealkali metal oxide, lithium oxide (Li₂O), sodium oxide (Na₂O), potassiumoxide (K₂O), and the like can be given. As the alkali earth metal oxide,magnesium oxide (MgO), calcium oxide (CaO), and the like can be given.As alkali metal fluoride, lithium fluoride (LiF), cesium fluoride (CsF),and the like can be given. As alkali earth metal fluoride, magnesiumfluoride (MgF₂), calcium fluoride (CaF₂), and the like can be given.Further, the second mixed layer contains an aromatic hydrocarbon and ametal oxide. The aromatic hydrocarbon is not particularly limited here;however, an aromatic hydrocarbon having hole mobility of 1×10⁻⁶ cm²/Vsor more is preferably used. As such an aromatic hydrocarbon, forexample, 2-tert-butyl-9,10-di(2-naphthyl)anthracene; anthracene;9,10-diphenylanthracene; tetracene; rubrene; perylene;2,5,8,11-tetra(tert-butyl)perylene; and the like can be given. As themetal oxide, a metal oxide showing an electron accepting property toaromatic hydrocarbon is preferable. As such a metal oxide, for example,molybdenum oxide, vanadium oxide, ruthenium oxide, rhenium oxide, andthe like can be given. The aromatic hydrocarbon and the metal oxide arepreferably mixed in the layer so that when the layer containing thesesubstances is measured by an electron spin resonance technique, thelayer shows a peak derived from resonance of unpaired electrons, andtransmittance in a spectrum of 450 to 650 nm is 80% or more, andconcretely, 80 to 95%, or absorbance per 1 μm in the spectrum of 450 to650 nm is 1 or less, and concretely, 0.3 to 0.8.

In another aspect of the present invention, a light emitting elementincludes a layer containing an aromatic hydrocarbon and a metal oxidebetween a pair of electrodes. The aromatic hydrocarbon is notparticularly limited; however, an aromatic hydrocarbon having holemobility of 1×10⁻⁶ cm²/Vs or more (and more preferably, 1×10⁻⁶ to 1×10⁰cm²/Vs) is preferably used. As such an aromatic hydrocarbon havingfavorable hole mobility, for example, aromatic hydrocarbons having 14 to60 carbon atoms and containing an anthracene skeleton such as2-tert-butyl-9,10-di(2-naphthyl)anthracene;9,10-di(naphthalen-1-yl)-2-tert-butylanthracene; anthracene;9,10-diphenylanthracene; 9,10-bis(3,5-diphenylphenyl)anthracene;9,10-di(naphthalen-2-yl)anthracene; 2-tent-butylanthracene,9,10-di(4-methylnaphthalen-1-yl)anthracene;9,10-bis[2-(naphthalen-1-yl)phenyl]anthracene;2,3,6,7-tetramethyl-9,10-di(naphthalene -1-yl)anthracene;2,3,6,7-tetramethyl-9,10-di(naphthalen-2-yl)anthracene; bianthryl;10,10′-di(2-phenylphenyl)-9,9′-bianthryl;10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′ -bianthryl; and9,10-di(4-phenylphenyl)-2-tert-butylanthracene, can be given. Among thearomatic hydrocarbons having 14 to 60 carbon atoms, in particular, anaromatic hydrocarbon having 26 to 60 carbon atoms is preferably used.More preferably, an aromatic hydrocarbon having 34 to 60 carbon atoms isused. As the metal oxide, a metal oxide showing an electron acceptingproperty to an aromatic hydrocarbon is preferable. As such a metaloxide, for example, molybdenum oxide, vanadium oxide, ruthenium oxide,rhenium oxide, and the like can be given. The aromatic hydrocarbon andthe metal oxide are preferably mixed in the layer so that when the layercontaining these substances is measured by an electron spin resonancetechnique, the layer shows a peak derived from resonance of unpairedelectrons, and transmittance in a spectrum of 450 to 650 nm is 80% ormore, and concretely, 80 to 100%.

In another aspect of the present invention, a light emitting elementincludes a light emitting layer between a first electrode and a secondelectrode, and a layer containing an aromatic hydrocarbon and a metaloxide between the light emitting layer and the first electrode. Whenvoltage is applied to each of the first and second electrodes so thatpotential of the first electrode is higher than that of the secondelectrode, a light emitting substance contained in the light emittinglayer emits light. The aromatic hydrocarbon is not particularly limited;however, an aromatic hydrocarbon having hole mobility of 1×10⁻⁶ cm²/Vsor more (more preferably, 1×10⁻⁶ to 1×10⁰ cm²/Vs) is preferably used. Assuch an aromatic hydrocarbon having favorable hole mobility, forexample, aromatic hydrocarbons having 14 to 60 carbon atoms andcontaining an anthracene skeleton such as2-tent-butyl-9,10-di(2-naphthyl)anthracene;9,10-di(naphthalen-1-yl)-2-tert-butyl anthracene; anthracene;9,10-diphenylanthracene; 9,10bis(3, 5-diphenylphenyl)anthracene;9,10-di(naphthalen-2-yl)anthracene; 2-tert-butylanthracene;9,10-di(4-methylnaphthalen-1-yl)anthracene;9,10-bis[2-(naphthalen-1-yl)phenyl]anthracene;2,3,6,7-tetramethyl-9,10-di(naphthalen-1-yl)anthracene;2,3,6,7-tetramethyl-9,10-di(naphthalen-2-yl)anthracene; bianthryl;10,10′-di(2-phenylphenyl)-9,9′-bianthryl;10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl; and9,10-di(4-phenylphenyl)-2-tert-butylanthracene, can be given. Amongaromatic hydrocarbons having 14 to 60 carbon atoms, in particular, anaromatic hydrocarbon having 26 to 60 carbon atoms is preferably used.More preferably, an aromatic hydrocarbon having 34 to 60 carbon atoms isused. As the metal oxide, a metal oxide showing an electron acceptingproperty to an aromatic hydrocarbon is preferable. As such a metaloxide, for example, molybdenum oxide, vanadium oxide, ruthenium oxide,rhenium oxide, and the like can be given. The aromatic hydrocarbon andthe metal oxide are preferably mixed in the layer so that when the layercontaining these substances is measured by an electron spin resonancetechnique, the layer shows a peak derived from resonance of unpairedelectrons, and transmittance in a spectrum of 450 to 650 nm is 80% ormore, and concretely, 80 to 100%.

In another aspect of the present invention, a light emitting elementincludes a light emitting layer, a first mixed layer, and a second mixedlayer between a first electrode and a second electrode, wherein whenvoltage is applied to each of the first and second electrodes so thatpotential of the first electrode is higher than that of the secondelectrode, a light emitting substance contained in the light emittinglayer emits light. In the light emitting element, the light emittinglayer is provided to be closer to the first electrode than the firstmixed layer. The second mixed layer is provided to be closer to thesecond electrode than the first mixed layer. The first mixed layercontains a substance selected from the group consisting of an alkalimetal, an alkali earth metal, an alkali metal oxide, an alkali earthmetal oxide, an alkali metal fluoride, and an alkali earth metalfluoride; and a substance having an electron transporting property. Asthe alkali metal, lithium (Li), sodium (Na), potassium (K), and the likecan be given here, for example. As the alkali earth metal, magnesium(Mg), calcium (Ca), and the like can be given, for example. As thealkali metal oxide, lithium oxide (Li₂O), sodium oxide (Na₂O), potassiumoxide (K₂O), and the like can be given. As the alkali earth metal oxide,magnesium oxide (MgO), calcium oxide (CaO), and the like can be given.As the alkali metal fluoride, lithium fluoride (LiF), cesium fluoride(CsF), and the like can be given. As the alkali earth metal fluoride,magnesium fluoride (MgF₂), calcium fluoride (CaF₂), and the like can begiven. Further, the second mixed layer contains an aromatic hydrocarbonand a metal oxide. The aromatic hydrocarbon is not particularly limitedhere; however, an aromatic hydrocarbon having hole mobility of 1×10⁻⁶cm²/Vs or more is preferably used. As such an aromatic hydrocarbonhaving favorable hole mobility, for example, aromatic hydrocarbonshaving 14 to 60 carbon atoms and containing an anthracene skeleton suchas 2-tert-butyl-9,10-di(2-naphthyl)anthracene;9,10-di(naphthalen-1-yl)-2-tert-butylanthracene; anthracene;9,10-diphenylanthracene; 9,10-bis(3,5-diphenylphenyl)anthracene;9,10-di(naphthalen-2-yl)anthracene; 2-tert-butylanthracene;9,10-di(4-methylnaphthalen-1-yl)anthracene;9,10-bis[2-(naphthalen-1-yl)phenyl]anthracene;2,3,6,7-tetramethyl-9,10-di(naphthalen-1-yl)anthracene;2,3,6,7-tetramethyl-9,10-di(naphthalen-2-yl)anthracene; bianthryl;10,10′-di(2-phenylphenyl)-9,9′-bianthryl;10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl; and9,10-di(4-phenylphenyl)-2-tert-butylanthracene, can be given. Amongaromatic hydrocarbons having 14 to 60 carbon atoms, in particular, anaromatic hydrocarbon having 26 to 60 carbon atoms is preferably used.More preferably, an aromatic hydrocarbon having 34 to 60 carbon atoms isused. As the metal oxide, a metal oxide showing an electron acceptingproperty to an aromatic hydrocarbon is preferable. As such a metaloxide, for example, molybdenum oxide, vanadium oxide, ruthenium oxide,rhenium oxide, and the like can be given. The aromatic hydrocarbon andthe metal oxide are preferably mixed in the layer so that when the layercontaining these substances is measured by an electron spin resonancetechnique, the layer shows a peak derived from resonance of unpairedelectrons, and transmittance in a spectrum of 450 to 650 nm is 80% ormore, and concretely, 80 to 100%.

In another aspect of the present invention, a light emitting elementincludes n (n is a given natural number) pieces of light emitting layersbetween a first electrode and a second electrode; and a first mixedlayer and a second mixed layer between an m-th light emitting layer (mis a given natural number of 1 ≦m ≦n) and an (m +1)-th light emittinglayer, wherein when voltage is applied to each of the first and secondelectrodes so that potential of the first electrode is higher than thatof the second electrode, a light emitting substance contained in thelight emitting layer emits light. The first mixed layer is provided tobe closer to the first electrode than the second mixed layer. The firstmixed layer contains a substance selected from the group consisting ofan alkali metal, an alkali earth metal, an alkali metal oxide, an alkaliearth metal oxide, an alkali metal fluoride, and an alkali earth metalfluoride; and a substance having an electron transporting property. Asthe alkali metal, lithium (Li), sodium (Na), potassium (K), and the likecan be given here, for example. As the alkali earth metal, magnesium(Mg), calcium (Ca), and the like can be given, for example. As thealkali metal oxide, lithium oxide (Li₂O), sodium oxide (Na₂O), potassiumoxide (K₂O), and the like can be given. As the alkali earth metal oxide,magnesium oxide (MgO), calcium oxide (CaO), and the like can be given.As the alkali metal fluoride, lithium fluoride (LiF), cesium fluoride(CsF), and the like can be given. As the alkali earth metal fluoride,magnesium fluoride (MgF₂), calcium fluoride (CaF₂), and the like can begiven. Further, the second mixed layer contains an aromatic hydrocarbonand a metal oxide. The aromatic hydrocarbon is not particularly limitedhere; however, an aromatic hydrocarbon having hole mobility of 1×10⁻⁶cm²/Vs or more is preferably used. As such an aromatic hydrocarbonhaving favorable hole mobility, for example, aromatic hydrocarbonshaving 14 to 60 carbon atoms and containing an anthracene skeleton suchas 2-tert-butyl-9,10-di(2-naphthyl)anthracene;9,10-di(naphthalen-1-yl)-2-tert-butylanthracene; anthracene;9,10-diphenylanthracene; 9,10-bis(3,5-diphenylphenyl)anthracene;9,10-di(naphthalen-2-yl)anthracene; 2-tert-butylanthracene;9,10-di(4-methylnaphthalen-1-yl)anthracene;9,10-bis[2-(naphthalen-1-yl)phenyl]anthracene;2,3,6,7-tetramethyl-9,10-di(naphthalen-1-yl)anthracene;2,3,6,7-tetramethyl-9,10-di(naphthalen-2-yl)anthracene; bianthryl;10,10′-di(2-phenylphenyl)-9,9′-bianthryl;10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl; and9,10-di(4-phenylphenyl)-2-tert-butylanthracene, can be given. Amongaromatic hydrocarbons having 14 to 60 carbon atoms, in particular, anaromatic hydrocarbon having 26 to 60 carbon atoms is preferably used.More preferably, an aromatic hydrocarbon having 34 to 60 carbon atoms isused. As the metal oxide, a metal oxide showing an electron acceptingproperty to an aromatic hydrocarbon is preferable. As such a metaloxide, for example, molybdenum oxide, vanadium oxide, ruthenium oxide,rhenium oxide, and the like can be given. The aromatic hydrocarbon andthe metal oxide are preferably mixed in the layer so that when the layercontaining these substances is measured by an electron spin resonancetechnique, the layer shows a peak derived from resonance of unpairedelectrons, and transmittance in a spectrum of 450 to 650 nm is 80% ormore, and concretely, 80 to 100%.

In another aspect of the present invention, a light emitting elementincludes a layer containing an aromatic hydrocarbon and a metal oxidebetween a pair of electrodes. The aromatic hydrocarbon is notparticularly limited; however, an aromatic hydrocarbon having holemobility of 1×10⁻⁶ cm²/Vs or more (more preferably, 1×10⁻⁶ to 1×10⁰cm²/Vs) is preferably used. As such an aromatic hydrocarbon havingfavorable hole mobility, for example, aromatic hydrocarbons having 14 to60 carbon atoms and containing an anthracene skeleton such as2-tert-butyl-9,10-di(2-naphthyl)anthracene;9,10-di(naphthalen-1-yl)-2-tert-butyl anthracene; anthracene;9,10-diphenylanthracene; 9,10-bis(3,5-diphenylphenyl)anthracene;9,10-di(naphthalen-2-yl)anthracene; 2-tert-butylanthracene;9,10-di(4-methylnaphthalen-1-yl)anthracene;9,10-bis[2-(naphthalen-1-y1)phenyl]anthracene;2,3,6,7-tetramethyl-9,10-di(naphthalen-1-yl)anthracene;2,3,6,7-tetramethyl-9,10-di(naphthalen-2-yl)anthracene; bianthryl;10,10′-di(2-phenylphenyl)-9,9′-bianthryl;10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl; and9,10-di(4-phenylphenyl)-2-tert-butylanthracene, can be given. Amongaromatic hydrocarbons having 14 to 60 carbon atoms, in particular, anaromatic hydrocarbon having 26 to 60 carbon atoms is preferably used.More preferably, an aromatic hydrocarbon having 34 to 60 carbon atoms isused. As the metal oxide, a metal oxide showing an electron acceptingproperty to an aromatic hydrocarbon is preferable. As such a metaloxide, for example, molybdenum oxide, vanadium oxide, ruthenium oxide,rhenium oxide, and the like can be given. The aromatic hydrocarbon andthe metal oxide are preferably mixed in the layer so that when the layercontaining these substances is measured by an electron spin resonancetechnique, the layer shows a spectrum with a g-value of 2.0027 to2.0030, and transmittance in a spectrum of 450 to 650 nm is 80% or more,and concretely, 80 to 100%.

In another aspect of the present invention, a light emitting deviceincludes any of the light emitting elements described above.

In another aspect of the present invention, an electronic appliance usesa light emitting device, which includes any of the light emittingelements described above as a pixel, for a display portion.

In another aspect of the present invention, an electronic appliance usesa light emitting device, which includes any of the light emittingelements described above as a light source, for a lighting portion.

By implementing the present invention, a light emitting element whose anoperational defect due to crystallization of a layer provided between apair of electrodes is reduced, can be obtained. Since an aromatichydrocarbon and a metal oxide are mixed in the layer, a layer which isnot easily crystallized can be formed.

By implementing the present invention, a light emitting element, inwhich the length of a light path through which emitted light passes canbe easily changed, can be obtained. This is because providing a mixedlayer containing an aromatic hydrocarbon and a metal oxide between apair of electrodes makes it possible to obtain a light emitting elementrequiring less driving voltage, which is dependent on increase inthickness of the mixed layer. As a result, a distance between the lightemitting layer and one of the electrodes can be easily changed.

By implementing the present invention, a light emitting element, inwhich short-circuiting is not easily caused between a pair ofelectrodes, can be obtained. Since a mixed layer containing an aromatichydrocarbon and a metal oxide is provided between the pair ofelectrodes, a light emitting element requiring extremely low derivingvoltage, which is dependent on increase in thickness of the mixed layer,can be obtained. As a result, by increasing the thickness of the mixedlayer, concavity and convexity generated on a surface (i.e., unflatnessof a surface) of one of the electrodes can be easily reduced.

By implementing the present invention, a light emitting element, inwhich variations in light extraction efficiency caused depending on anemission wavelength can be prevented, can be obtained. Since a mixedlayer, which contains an aromatic hydrocarbon and a metal oxide used inthe present invention has a characteristic of less variations inabsorbance, which is dependent on a wavelength, even if light emissionsare differed in wavelength, the amount of light emission loss caused bybeing absorbed in the mixed layer provided between the light emittinglayer and one of the electrodes is not so much differed.

By implementing the present invention, a light emitting element whichemits light with high color purity can be obtained, thereby obtaining alight emitting device being capable of providing images with favorablecolors. This is because the length of the light path through which lightpasses can be easily changed and adjusted to be a suitable lengthwithout increasing driving voltage.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram explaining one mode of a light emitting element ofthe present invention;

FIG. 2 is a diagram explaining one mode of a light emitting element ofthe present invention;

FIG. 3 is a diagram explaining one mode of a light emitting element ofthe present invention;

FIG. 4 is a diagram explaining one mode of a light emitting element ofthe present invention;

FIG. 5 is a diagram explaining one mode of a light emitting element ofthe present invention;

FIG. 6 is a diagram explaining one mode of a light emitting element ofthe present invention;

FIG. 7 is a diagram explaining one mode of a light emitting element ofthe present invention;

FIG. 8 is a diagram explaining one mode of a light emitting element ofthe present invention;

FIG. 9 is a diagram explaining one mode of a light emitting element ofthe present invention;

FIG. 10 is a diagram explaining one mode of a light emitting element ofthe present invention;

FIG. 11 is a diagram explaining one mode of a light emitting element ofthe present invention;

FIG. 12 is a diagram explaining one mode of a light emitting element ofthe present invention;

FIG. 13 is a diagram explaining one mode of a light emitting element ofthe present invention;

FIG. 14 is a diagram explaining one mode of a light emitting element ofthe present invention;

FIG. 15 is a top view explaining one mode of a light emitting element ofthe present invention;

FIG. 16 is a diagram explaining one mode of a circuit for driving apixel provided in a light emitting device of the present invention;

FIG. 17 is a diagram explaining one mode of a pixel portion included ina light emitting device of the present invention;

FIG. 18 is a frame diagram explaining a method for driving a pixelincluded in a light emitting device of the present invention;

FIGS. 19A to 19C are cross sectional views explaining modes of crosssections of light emitting devices of the present invention;

FIG. 20 is a diagram explaining one mode of a light emitting device ofthe present invention;

FIGS. 21A to 21C are diagrams explaining modes of electronic appliancesto which the present invention is applied;

FIG. 22 is a diagram explaining a lighting device to which the presentinvention is applied;

FIG. 23 is a diagram explaining a method for manufacturing a lightemitting device of Embodiment 1;

FIG. 24 is a graph showing voltage-luminance characteristics of a lightemitting element of Embodiment 1;

FIG. 25 is a graph showing voltage-current characteristics of a lightemitting element of Embodiment 1;

FIG. 26 is a graph showing luminance-current efficiency characteristicsof a light emitting element of Embodiment 1;

FIG. 27 is a graph showing voltage-current characteristics of a lightemitting element of Embodiment 2;

FIG. 28 is a graph showing an absorption spectrum of an element ofEmbodiment 3;

FIG. 29 is a graph showing absorbance per 1 μm of an element ofEmbodiment 3;

FIG. 30 is a graph showing transmittance of an element of Embodiment 3;

FIG. 31 is a graph showing an ESR spectrum of a sample 7;

FIG. 32 is a graph showing an ESR spectrum of a sample 8;

FIG. 33 is a graph showing wavelength dependence of transmittance ofsamples 9 and 10;

FIG. 34 is a graph showing wavelength dependence of transmittance ofsamples 11 to 14;

FIG. 35 is a graph showing wavelength dependence of transmittance ofsamples 15 to 17;

FIG. 36 is a graph showing wavelength dependence of transmittance ofsamples 18 to 20;

FIG. 37 is a graph showing voltage-current characteristics of a sample21;

FIG. 38 is a graph showing voltage-luminance characteristics of lightemitting elements 3 to 5;

FIG. 39 is a graph showing voltage-current characteristics of the lightemitting elements 3 to 5;

FIG. 40 is a graph showing luminance-current efficiency characteristicsof the light emitting elements 3 to 5;

FIG. 41 is a graph showing luminance-electric power efficiencycharacteristics of the light emitting elements 3 to 5;

FIG. 42 is a graph showing luminance-electric power efficiencycharacteristics of light emitting elements 4, 6, and 7;

FIGS. 43A and 43B are graphs showing changes in luminance withaccumulation of light emitting time and changes in driving voltage withaccumulation of light emitting time of the light emitting elements 4, 6,and 7;

FIGS. 44A and 44B are graphs showing changes in luminance withaccumulation of light emitting time and changes in driving voltage withaccumulation of light emitting time of light emitting elements 8 and 9;

FIG. 45 is a graph showing voltage-luminance characteristics of lightemitting elements 10 to 12; and

FIG. 46 is a graph showing voltage-luminance characteristics of lightemitting elements 13 to 15.

DETAILED DESCRIPTION OF THE INVENTION

Embodiment Modes

The embodiment modes according to the present invention will hereinafterbe described with reference to the accompanying drawings. It is easilyunderstood by those skilled in the art that the embodiment modes anddetails herein disclosed can be modified in various ways withoutdeparting from the purpose and the scope of the present invention. Thepresent invention should not be interpreted as being limited to thedescription of the embodiment modes to be given below.

Embodiment Mode 1

One embodiment mode of a light emitting element of the present inventionwill be described with reference to FIG. 1.

FIG. 1 shows a light emitting element having a light emitting layer 113between a first electrode 101 and a second electrode 102. In the lightemitting element shown in FIG. 1, a mixed layer 111 is provided betweenthe light emitting layer 113 and the first electrode 101. A holetransporting layer 112 is provided between the light emitting layer 113and the mixed layer 111. An electron transporting layer 114 and anelectron injecting layer 115 are provided between the light emittinglayer 113 and the second electrode 102. In this light emitting element,when voltage is applied to both the first electrode 101 and the secondelectrode 102 so that potential of the first electrode 101 is higherthan that of the second electrode 102, holes are injected to the lightemitting layer 113 from the side of the first electrode 101 whereaselectrons are injected to the light emitting layer 113 from the side ofthe second electrode 102. Then, the holes and electrons injected to thelight emitting layer 113 are recombined. The light emitting layer 113contains a light emitting substance. The light emitting substance isexcited by excitation energy generated by recombination of the holes andelectrons. The light emitting substance in an excited state emits lightupon returning to a ground state.

The first electrode 101, the second electrode 102, and each layerprovided between the first electrode 101 and the second electrode 102will be described in detail below.

A substance used for forming the first electrode 101 is not particularlylimited. A substance having a low work function such as aluminum andmagnesium can be used in addition to a substance having a high workfunction such as indium tin oxide, indium tin oxide containing siliconoxide, indium oxide containing 2 to 20 wt % zinc oxide, gold (Au),platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), or tantalumnitride. This is because holes are generated from the mixed layer 111when applying voltage to the light emitting element of the presentinvention.

The second electrode 102 is preferably formed using a substance having alow work function such as aluminum and magnesium. When a layergenerating electrons is provided between the second electrode 102 andthe light emitting layer 113, however, a substance having a high workfunction such as indium tin oxide, indium tin oxide containing siliconoxide, indium oxide containing 2 to 20 wt % zinc oxide, gold (Au),platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), or tantalumnitride; and the like can also be used. Therefore, the second electrode102 may be formed by appropriately selecting either a substance having ahigh work function or a substance having a low work function inaccordance with a characteristic of a layer provided between the secondelectrode 102 and the light emitting layer 113.

Note that one or both the first electrode 101 and the second electrode102 is/are preferably formed so that one or both of the electrodes cantransmit light.

The mixed layer 111 contains an aromatic hydrocarbon and a metal oxide.The aromatic hydrocarbon is not particularly limited; however, anaromatic hydrocarbon having hole mobility of 1×10⁻⁶ cm²/Vs or more (morepreferably, 1×10⁻⁶ to 1×10⁰ cm²/Vs) is preferably used. When thearomatic hydrocarbon has hole mobility of 1×10⁻⁶ cm²/Vs or more, holesinjected from the metal oxide can be efficiently transported. As such anaromatic hydrocarbon having hole mobility of 1×10⁻⁶ cm²/Vs or more, forexample, 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation:t-BuDNA); anthracene; 9,10-diphenylanthracene; tetracene; rubrene;perylene; 2,5,8,11-tetra(tert-butyl)perylene; and the like can be given.In addition, pentacene, coronene, and the like can be used. An aromatichydrocarbon having hole mobility of 1×10⁻⁶ cm²/Vs or more and having 14to 42 carbon atoms such as listed aromatic hydrocarbons is preferablyused. As the metal oxide, a metal oxide showing an electron acceptingproperty to an aromatic hydrocarbon is preferable. As such a metaloxide, for example, molybdenum oxide, vanadium oxide, ruthenium oxide,rhenium oxide, and the like can be given. In addition, metal oxides suchas titanium oxide, chromium oxide, zirconium oxide, hafnium oxide,tantalum oxide, tungsten oxide, or silver oxide can also be used. Bymixing the metal oxide and the aromatic hydrocarbon as mentioned above,charge-transfer can be generated, which results in generation of acharge-transfer complex. The mixed layer 111 in which a charge-transfercomplex is generated, contains unpaired electrons. Further, as comparedwith a mixed film, in which divanadium pentoxide and α-NPD are mixed,disclosed in the related art as described above (Japanese PatentApplication Laid-Open No. 2005-123095) and a mixed film in whichdirhenium heptaoxide and α-NPD are mixed, the mixed layer 111 containinga metal oxide and an aromatic hydrocarbon shows extremely lessvariations in absorbance, which is dependent on an absorption wavelengthin a visible light region which is a region of 450 to 650 nm (i.e., lessvariations in transmittance in a visible light region), and therefore,the amount of light emission loss caused by absorption of light emissionin the mixed layer 111 is less dependent on a light emission wavelength.Therefore, providing the mixed layer 111 makes it possible to preventlight extraction efficiency from varying for each color (i.e., for eachwavelength) of light emission. The metal oxide is preferably containedin the mixed layer 111 so that a mass ratio of the metal oxide withrespect to the aromatic hydrocarbon (=metal oxide/aromatic hydrocarbon)is 0.5 to 2 or a molar ratio thereof is 1 to 4. When the mixed layer 111contains the metal oxide and the aromatic hydrocarbon with such amixture ratio, the transmittance of light in a spectrum of 450 to 650 nmcan be set to be 80% or more, and specifically, 80 to 95%, in the mixedlayer 111. Such the transmittance allows a light emitting element toextract light emission more efficiently. Further, aromatic hydrocarbonshave generally a property of being easily crystallized; however, when anaromatic hydrocarbon is mixed with a metal oxide like this embodimentmode, the aromatic hydrocarbon is not easily crystallized. Furthermore,molybdenum oxide is particularly crystallized easily among metal oxides;however, when molybdenum oxide is mixed with an aromatic hydrocarbonlike this embodiment mode, the molybdenum oxide is not easilycrystallized. By mixing the aromatic hydrocarbon and the metal oxide inthis manner, the aromatic hydrocarbon and the metal oxide mutuallyhinder crystallization, in consequence, a layer which is not easilycrystallized can be obtained. Moreover, since aromatic hydrocarbons havea high glass transition temperature, when the mixed layer 111 containingan aromatic hydrocarbon is applied as a mixed layer like this embodimentmode, a mixed layer having a more excellent heat resistance propertyalong with a function of more favorably injecting holes also to the holetransporting layer 112 can be obtained as compared with a hole injectinglayer formed using 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA);4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA); 4,4′-bis{N-[4-(N,N-di-m-tolylamino)phenyl]-N-phenylamino}biphenyl (abbreviation:DNTPD); or the like. With respect to a glass transition temperature oft-BuDNA, which is one of aromatic hydrocarbons used in the presentinvention for reference, a glass transition temperature of t-BuDNA is127° C., and it is know that the glass transition temperature is higherthan 98° C. of a glass transition temperature of α-NPD, which isdescribed in the patent document 1.

The hole transporting layer 112 has a function of transporting holes. Inthe light emitting element of this embodiment mode, the holetransporting layer 112 has a function of transporting holes to the lightemitting layer 113 from the mixed layer 111. Providing the holetransporting layer 112 makes it possible to create a distance betweenthe mixed layer 111 and the light emitting layer 113. Consequently, itis possible to prevent quenching of light emission due to metalcontained in the mixed layer 111. The hole transporting layer 112 ispreferably formed using a substance having a hole transporting property.In particular, the hole transporting layer 112 is preferably formedusing a substance having a hole transporting property with hole mobilityof 1×10⁻⁶ cm²/Vs or more (more preferably, 1×10⁻⁶ to 1×10⁰ cm²/Vs), or abipolar substance. Note that the substance having the hole transportingproperty is a substance having higher hole mobility than electronmobility, and preferably corresponds to a substance in which a ratio ofhole mobility to electron mobility (i.e., hole mobility/electronmobility) is larger than 100. As specific examples of the substancehaving the hole transporting property,4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB);4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl (abbreviation: TPD);1,3,5-tris[N,N-di(m-tolyl)amino]benzene (abbreviation: m-MTDAB);4,4′,4″-tris(N-carbazolyl)triphenylamine (abbreviation: TCTA); and thelike can be given. Further, the bipolar substance is a substance inwhich a ratio of mobility of one carrier to mobility of the othercarrier is 100 or less, and preferably, 10 or less. As the bipolarsubstance, for example, 2,3-bis(4-diphenylaminophenyl)quinoxaline(abbreviation: TPAQn);2,3-bis{4-[N-(1-naphthyl)-N-phenylamino]phenyl}-dibenzo[f,h]quinoxaline(abbreviation: NPADiBzQn); and the like can be given. Among bipolarsubstances, in particular, a bipolar substance having mobility of holesand electrons of 1×10⁻⁶ cm²/Vs or more (more preferably, 1×10⁻⁶ to 1×10⁰cm²/Vs), is preferably used.

The light emitting layer 113 contains a light emitting substance. Thelight emitting substance is a substance having favorable light emittingefficiency which can emit light with a desired wavelength. The lightemitting layer 113 may be formed using only a light emitting substance.When quenching due to a concentration is caused, the light emittinglayer 113 is preferably a layer in which a light emitting substance isdispersed in a substance having a larger energy gap than that of thelight emitting substance. By dispersing the light emitting substance inthe light emitting layer 113, quenching of light emission due to aconcentration can be prevented. Here, an energy gap indicates an energygap between a LUMO level and a HOMO level.

The light emitting substance is not particularly limited, and asubstance with favorable light emitting efficiency, which can emit lightwith a desired light emission wavelength may be used. For example, inorder to obtain red light emission, for example, the followingsubstances showing emission spectrum with peaks in a spectrum of 600 to680 nm can be employed as the light emitting substance:4-dicyanomethylene-2-isopropyl-6-[2-(1,1,7,7-tetramethyl-9-julolidyl)ethenyl]-4H-pyran(abbreviation: DCJTI);4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyl-9-julolidyl)ethenyl]-4H-pyran(abbreviation: DCJT);4-dicyanomethylene-2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-9-julolidyl)ethenyl]-4H-pyran(abbreviation: DCJTB); periflanthene;2,5-dicyano-1,4-bis(2-[10-methoxy-1,1,7,7-tetramethyl-9-julolidyl]ethenyl)benzeneand the like. In order to obtain green light emission, a substanceshowing emission spectrum with peaks in a spectrum of 500 to 550 nm suchas N,N′-dimethylquinacridon (abbreviation: DMQd), coumarin 6, coumarin545T, or tris(8-quinolinolato)aluminum (abbreviation: Alq₃) can beemployed as the light emitting substance. In order to obtain blue lightemission, the following substances showing emission spectrum with peaksin a spectrum of 420 to 500 nm can be employed as the light emittingsubstance: 9,10-bis(2-naphthyl)-tert-butylanthracene (abbreviation:t-BuDNA); 9,9′-bianthryl; 9,10-diphenylanthracene (abbreviation: DPA);9,10-bis(2-naphthyl)anthracene (abbreviation: DNA);bis(2-methyl-8-quinolinolato)-4-phenylphenolato-gallium (abbreviation:BGaq); bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum(abbreviation: BAlq); and the like. In addition to the above mentionedsubstances, which emit fluorescence, the following substances, whichemit phosphorescence, can be used as the light emitting material:bis[2-(3,5-bis(trifluoromethyl)phenyl)pyridinato-N,C²′]iridium(III)picolinate(abbreviation: Ir(CF₃ppy)₂(pic));bis[2-(4,6-difluorophenyl)pyridinato-N,C²′]iridium(III) acetylacetonate(abbreviation: FIr(acac));bis[2-(4,6-difluorophenyl)pyridinato-N,C²′]iridium(III) picolinate(abbreviation: FIr(pic)); tris(2-phenylpyridinato-N,C²′) iridium(abbreviation: Ir(ppy)₃); and the like.

A substance (also referred to as a host) used for dispersing a lightemitting substance, which is contained in the light emitting layer 113along with the light emitting substance is not particularly limited, andmay be appropriately selected in consideration for an energy gap and thelike of a substance used for the light emitting substance. For example,an anthracene derivative such as9,10-di(2-naphthyl)-2-tert-butylanthracene (abbreviation: t-BuDNA); acarbazole derivative such as 4,4′-bis(N-carbazolyl)biphenyl(abbreviation: CBP); a quinoxaline derivative such as2,3-bis(4-diphenylaminophenyl) quinoxaline (abbreviation: TPAQn), or2,3-bis{4-[N-(1-naphthyl)-N-phenylamino]phenyl}-dibenzo[f,h]quinoxaline(abbreviation: NPADiBzQn); a metal complex such asbis[2-(2-hydroxyphenyl)pyridinato]zinc (abbreviation: Znpp₂), orbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂); andthe like can be used. Note that a light emitting substance is referredto as a guest with respect to the host.

The electron transporting layer 114 has a function of transportingelectrons. In the light emitting element of this embodiment mode, theelectron transporting layer 114 has a function of transporting electronsinjected from the second electrode 102 side to the light emitting layer113. Providing the electron transporting layer 114 makes it possible tocreate a distance between the second electrode 102 and the lightemitting layer 113. Consequently, quenching of light emission due tometal contained in the second electrode 102 can be prevented. Theelectron transporting layer is preferably formed using a substancehaving an electron transporting property. In particular, the electrontransporting layer is preferably formed using a substance having anelectron transporting property with electron mobility of 1×10⁻⁶ cm²/Vsor more (more preferably, 1×10⁻⁶ to 1×10⁰ cm²/Vs), or a bipolarsubstance. Further, the substance having the electron transportingproperty is a substance having higher electron mobility than holemobility, and corresponds to a substance in which a ratio of electronmobility to hole mobility (i.e., electron mobility/hole mobility) islarger than 100. As specific examples of the substance having theelectron transporting property, a metal complex such astris(8-quinolinolato)aluminum (abbreviation: Alq₃),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviation:BAlq), bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation:Zn(BOX)₂), or bis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation:Zn(BTZ)₂); 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole(abbreviation: PBD);1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviation: OXD-7); TAZ; p-EtTAZ; BPhen; BCP;4,4′-bis(5-methylbenzoxazolyl-2-yl)stilbene (abbreviation: BzOS); andthe like can be given. Further, the bipolar substance is the same asdescribed above. Note that the electron transporting layer 114 and thehole transporting layer 112 may be formed using the same bipolarsubstance.

The electron injecting layer 115 has a function of helping injection ofelectrons to the electron transporting layer 114 from the secondelectrode 102. By providing the electron injecting layer 115, differencein electron affinity between the second electrode 102 and the electrontransporting layer 114 can be reduced, thereby injecting electronseasily. The electron injecting layer 115 is preferably formed using asubstance whose electron affinity is larger than that of a substanceused for forming the electron transporting layer 114 and smaller thanthat of a substance used for forming the second electrode 102, or asubstance whose energy band is curved when it is provided as a thin filmwith a thickness of about 1 to 2 nm between the electron transportinglayer 114 and the second electrode 102. As specific examples ofsubstances, which can be used for forming the electron injecting layer115, the following inorganic materials can be given: alkali metals suchas lithium (Li), and the like; alkali earth metal such as magnesium(Mg), and the like; alkali metal fluorides such as cesium fluoride(CsF), and the like; alkali earth metal fluorides such as calciumfluoride (CaF₂), and the like; alkali metal oxides such as lithium oxide(Li₂O), sodium oxide (Na₂O), potassium oxide (K₂O), and the like; alkaliearth metal oxides such as calcium oxide (CaO) and magnesium oxide (MgO)and the like. When these substances are formed as thin films, theirenergy bands can be curved and the injection of electrons can be carriedout easily, and therefore, these substances are preferable. In additionto the inorganic materials, organic materials, which can be used forforming the electron transporting layer 114 such as bathophenanthroline(abbreviation: BPhen), bathocuproin (abbreviation: BCP),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), and3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: TAZ), can be used to form the electron injecting layer115 by selecting a substance having higher electron affinity than thatof a substance used for forming the electron transporting layer 114 fromamong them. That is, the electron injecting layer 115 may be formed byselecting a substance having electron affinity which is relativelyhigher than that of the electron transporting layer 114 from substanceshaving electron transporting properties. Further, in a case of providingthe electron injecting layer 115, the second electrode 102 is preferablyformed using a substance having a low work function such as aluminum.

A substance having an electron transporting property used for theelectron transporting layer 114 and an aromatic hydrocarbon contained inthe mixed layer 111 in the light emitting element as described above areselected respectively so that when mobility of the substance having theelectron transporting property used for the electron transporting layer114 is compared with mobility of the aromatic hydrocarbon contained inthe mixed layer 111, a ratio of mobility of one of the substances havingthe electron transporting property and the aromatic hydrocarbon tomobility of the other substance is preferably set to be 1,000 or less.By selecting the respective substances in such a manner, recombinationefficiency in the light emitting layer can be improved.

Note that the light emitting element having the hole transporting layer112, the electron transporting layer 114, the electron injecting layer115, and the like in addition to the mixed layer 111 and the lightemitting layer 113 is shown in this embodiment mode; however, a mode ofthe light emitting element is not necessarily limited thereto. Forexample, as shown in FIG. 2, an electron generating layer 116 and thelike may be provided as substitute for the electron injecting layer 115.The electron generating layer 116 generates electrons, and can be formedby mixing at least one substance of one or more of a substance having anelectron transporting property and a bipolar substance, and a substanceshowing an electron donating property with respect to the abovesubstance. In this case, a substance having electron mobility of 1×10⁻⁶cm²/Vs or more (more preferably, 1× 10⁻⁶ to 1 ×10⁰ cm²/Vs) is preferablyused among the substance having the electron transporting property andthe bipolar substance. As the substance having the electron transportingproperty and the bipolar substance, the above mentioned substances canbe used, respectively. Further, as the substance showing the electrondonating property, at least one substance selected from the groupconsisting of alkali metals and alkali earth metals such as lithium(Li), calcium (Ca), sodium (Na), potassium (K), magnesium (Mg), and thelike can be used. at least one substance selected from the groupconsisting of alkali metal oxides, alkali earth metal oxides, alkalimetal fluorides, alkali earth metal fluorides, and the like such aslithium oxide (Li₂O), calcium oxide (CaO), sodium oxide (Na₂O),potassium oxide (K₂O), magnesium oxide (MgO), lithium fluoride (LiF),cesium fluoride (CsF), and the like can be used as the substance showingthe electron donating property.

Further, as shown in FIG. 3, a hole blocking layer 117 may be providedbetween the light emitting layer 113 and the electron transporting layer114. Providing the hole blocking layer 117 makes it possible to preventholes from flowing toward the second electrode 102 through the lightemitting layer 113, thereby improving recombination efficiency ofcarriers. In addition, excitation energy generated in the light emittinglayer 113 can be prevented from transferring to other layer such as theelectron transporting layer 114. The hole blocking layer 117 can beformed by selecting, in particular, a substance having larger ionizationpotential and larger excitation energy than that of a substance used forforming the light emitting layer 113 from among substances, which can beused for forming the electron transporting layer 114, such as BAlq,OXD-7, TAZ, and BPhen. That is, the hole blocking layer 117 may beformed by selecting a substance so that ionization potential of the holeblocking layer 117 is relatively larger than that of the electrontransporting layer 114. Similarly, a layer for blocking electrons fromflowing toward the second electrode 102 through the light emitting layer113 may be provided between the light emitting layer 113 and the holetransporting layer 112.

Note that an operator of the present invention may appropriately selectwhether or not the electron injecting layer 115, the electrontransporting layer 114, and the hole transporting layer 112 areprovided. For example, in a case where a defect such as an opticalquenching due to metal is not caused without providing the holetransporting layer 112, the electron transporting layer 114, and thelike, or in a case where injection of electrons can be preferablyperformed from the electrode without providing the electron injectinglayer 115, these layers are not necessarily required to be provided.

By forming the light emitting layer having the mixed layer 111containing an aromatic hydrocarbon and a metal oxide, defects due tocrystallization of a layer formed between a pair of electrodes can bereduced. More specifically, a short-circuiting between a pair ofelectrodes which is caused as a result of generation of concavity andconvexity due to the crystallization, can be reduced as compared with alight emitting element having a layer only containing an aromatichydrocarbon or a metal oxide. Further, since the mixed layer 111 cangenerate holes, by providing the mixed layer 111 containing an aromatichydrocarbon and a metal oxide, a light emitting element having lessvariations in driving voltage, which is dependent on the thickness ofthe mixed layer 111, can be obtained. As a consequence, a distancebetween the light emitting layer 113 and the first electrode 101 can beeasily adjusted by changing the thickness of the mixed layer 111. Thatis, the length of a light path through which emitted light passes (lightpath length), can be easily adjusted so that light emission can beefficiently extracted to an external portion or color purity of lightextracted to an external portion is improved. Further, by increasing thethickness of the mixed layer 111, the concavity and convexity on thesurface of the first electrode 101 can be reduced, thereby reducingshort-circuiting between the pair of electrodes.

The light emitting element described above may be formed by using amethod by which the second electrode 102 is formed after the mixed layer111, the hole transporting layer 112, the light emitting layer 113, theelectron transporting layer 114, the electron injecting layer 115, andthe like are stacked over the first electrode 101 in this order.Alternatively, the light emitting layer may be formed by using a methodby which the electron injecting layer 115, the electron transportinglayer 114, the light emitting layer 113, the hole transporting layer112, and the mixed layer 111 are stacked over the second electrode 102in this order. In a case where the mixed layer 111 is formed after theformation of the light emitting layer 113 like the latter method, evenwhen the first electrode 101 is formed by sputtering, the mixed layer111 serve as a protection layer, and hence, it is possible to form afavorable light emitting element in which damage due to sputtering of alayer formed using an organic compound such as the light emitting layer113 is not easily generated.

Embodiment Mode 2

One mode of a light emitting element of the present invention will bedescribed with reference to FIG. 4.

FIG. 4 shows a light emitting element having a light emitting layer 213,a first mixed layer 215, and a second mixed layer 216 between a firstelectrode 201 and a second electrode 202, in which the light emittinglayer 213 is provided to be closer to the first electrode 201 than thefirst mixed layer 215 and the second mixed layer 216 is provided to becloser to the second electrode 202 than the first mixed layer 215. Inthis light emitting element, a hole injecting layer 211 and a holetransporting layer 212 are provided between the light emitting layer 213and the first electrode 201, and an electron transporting layer 214 isprovided between the light emitting layer 213 and the first mixed layer215. The first mixed layer 215 contains at least one substance selectedfrom the group consisting of an alkali metal, an alkali earth metal, analkali metal oxide, an alkali earth metal oxide, an alkali metalfluoride, and an alkali earth metal fluoride; and a substance having anelectron transporting property. The second mixed layer 216 contains anaromatic hydrocarbon and a metal oxide. The light emitting layer 213contains a light emitting substance. When voltage is applied to each ofthe first electrode 201 and the second electrode 202 so that potentialof the first electrode 201 is higher than that of the second electrode202, electrons are injected to the electron transporting layer 214 fromthe first mixed layer 215, holes are injected to the second electrode202 from the second mixed layer 216, and holes are injected to the holeinjecting layer 211 from the first electrode 201. Further, holesinjected to the light emitting layer 213 from the first electrode 201side and electrons injected to the light emitting layer 213 from thesecond electrode 202 side are recombined. The light emitting substancecontained in the light emitting layer 213 is excited by excitationenergy generated by the recombination. The excited light emittingsubstance emits light upon returning to a ground state.

The first electrode 201, the second electrode 202, and each layerprovided between the first electrode 201 and the second electrode 202will be described in detail below.

The first electrode 201 is preferably formed using a substance having ahigh work function such as indium tin oxide, indium tin oxide containingsilicon oxide, indium oxide containing 2 to 20 wt % zinc oxide, gold(Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr),molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), ortantalum nitride.

The second electrode 202 is preferably formed using a substance having alow work function such as aluminum and magnesium. When a layergenerating electrons is provided between the second electrode 202 andthe light emitting layer 213, however, a substance having a high workfunction such as indium tin oxide, indium tin oxide containing siliconoxide, indium oxide containing 2 to 20 wt % zinc oxide, gold (Au),platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), or tantalumnitride; and the like can also be used. Therefore, the second electrode202 may be formed by appropriately selecting either a substance having ahigh work function or a substance having a low work function inaccordance with a characteristic of a layer provided between the secondelectrode 202 and the light emitting layer 213.

Note that one or both the first electrode 201 and the second electrode202 is/are preferably formed so that one or both the electrodes cantransmit light.

The hole injecting layer 211 of this embodiment mode has a function ofhelping injection of holes to the hole transporting layer 212 from thefirst electrode 201. Providing the hole injecting layer 211 makes itpossible to reduce difference in ionization potential between the firstelectrode 201 and the hole transporting layer 212 so that holes areeasily injected. The hole injecting layer 211 is preferably formed usinga substance whose ionization potential is smaller than that of asubstance used for forming the hole transporting layer 223 and largerthan that of a substance used for forming the first electrode 201. Asspecific examples of substances which can be used for forming the holeinjecting layer 211, a low molecular material such as phthalocyanine(abbreviation: H₂Pc) or copper phthalocyanine (abbreviation: CuPc), ahigh molecular material such as apoly(ethylenedioxythiophene)/poly(styrene sulfonate) solution(PEDOT/PSS), and the like can be given.

The hole transporting layer 212 has a function of transporting holes. Inthe light emitting element of this embodiment mode, the holetransporting layer 212 has a function of transporting holes to the lightemitting layer 213 from the hole injecting layer 211. Providing the holetransporting layer 212 makes it possible to create a distance betweenthe first electrode 201 and the light emitting layer 213. Consequently,it is possible to prevent quenching of light emission due to metalcontained in the first electrode 201. The hole transporting layer 212 ispreferably formed using a substance having a hole transporting property.In particular, the hole transporting layer 212 is preferably formedusing a substance having a hole transporting property with hole mobilityof 1×10⁻⁶ cm²/Vs or more or a bipolar substance. Further, the substancehaving the hole transporting property and the bipolar substance arealready described in Embodiment Mode 1, and will not be furtherdescribed in this embodiment mode.

The light emitting layer 213 contains a light emitting substance. Thelight emitting layer 213 may be formed using only a light emittingsubstance. When quenching due to a concentration is caused, the lightemitting layer 213 is preferably a layer in which a light emittingsubstance is dispersed in a substance having a larger energy gap thanthat of the light emitting substance. By mixing the light emittingsubstance in the light emitting layer 213, quenching of light emissiondue to a concentration can be prevented. Here, an energy gap indicatesan energy gap between a LUMO level and a HOMO level. The light emittingsubstance and the substance used for dispersing the light emittingsubstance are already described in Embodiment Mode 1, and will not befurther described in this embodiment mode.

The electron transporting layer 214 has a function of transportingelectrons. In the light emitting element of this embodiment mode, theelectron transporting layer 214 has a function of transporting electronsinjected from the first mixed layer 215 to the light emitting layer 213.Providing the electron transporting layer 214 makes it possible tocreate a distance between the second mixed layer 216 and the lightemitting layer 213. Consequently, quenching of light emission due tometal contained in the second mixed layer 216 (when metal is containedin the first mixed layer 215, quenching of light emission due to themetal) can be prevented. The electron transporting layer 214 ispreferably formed using a substance having an electron transportingproperty. In particular, the electron transporting layer is preferablyformed using a substance having an electron transporting property withelectron mobility of 1×10⁻⁶ cm²/Vs or more (more preferably, 1×10⁻⁶ to1×10⁰ cm²/Vs), or a bipolar substance. Further, the substance having theelectron transporting property and the bipolar substance are alreadydescribed in Embodiment Mode 1, and will no be further described in thisembodiment mode.

The first mixed layer 215 generates electrons. The first mixed layer 215can be formed by using a mixture of at least one substance of one ormore of a substance having an electron transporting property or abipolar substance and a substance showing an electron donating propertywith respect to the substance. In this case, a substance having electronmobility of 1×10⁻⁶ cm²/Vs or more (more preferably, 1×10⁻⁶ to 1×10⁰cm²/Vs) is preferably used among the substances having the electrontransporting properties and the bipolar substances. Further, thesubstance having the electron transporting property and the bipolarsubstance are already described in Embodiment Mode 1, and will not befurther described in this embodiment mode. Furthermore, the substanceshowing the electron donating property with respect to the substancehaving the electron transporting property and the bipolar substance isalso described in Embodiment Mode 1, and will not be further describedin this embodiment mode.

The second mixed layer 216 contains an aromatic hydrocarbon and a metaloxide. The aromatic hydrocarbon is not particularly limited; however, anaromatic hydrocarbon having hole mobility of 1×10⁻⁶ cm²/Vs or more ispreferably used. When the aromatic hydrocarbon has hole mobility of1×10⁻⁶ cm²/Vs or more (more preferably, 1×10⁻⁶ to 1×10⁰ cm²/Vs), holesinjected from the metal oxide can efficiently be transported. As such anaromatic hydrocarbon having hole mobility of 1×10⁻⁶ cm²/Vs or more, forexample, 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation:t-BuDNA); anthracene; 9,10-diphenylanthracene; tetracene; rubrene;perylene; 2,5,8,11-tetra(tert-butyl)perylene; and the like can be given.In addition, pentacene, coronene, and the like can also be used. Anaromatic hydrocarbon having hole mobility of 1×10⁻⁶ cm²/Vs or more and14 to 42 carbon atoms such as listed aromatic hydrocarbons is preferablyused. As the metal oxide, a metal oxide showing an electron acceptingproperty to an aromatic hydrocarbon is preferable. As such a metaloxide, for example, molybdenum oxide, vanadium oxide, ruthenium oxide,rhenium oxide, and the like can be given. In addition, metal oxides suchas titanium oxide, chromium oxide, zirconium oxide, hafnium oxide,tantalum oxide, tungsten oxide, or silver oxide can also be used. Bymixing the metal oxide and the aromatic hydrocarbon as mentioned above,charge-transfer can be generated. The second mixed layer 216 containinga metal oxide and an aromatic hydrocarbon shows extremely lessvariations in absorbance, which is dependent on an absorption wavelengthin a visible light region which is a region of 450 to 650 nm (i.e., lessvariations in transmittance in a visible light region), and therefore,the amount of light emission loss caused by absorption of light emissionin the second mixed layer 216 is less dependent on a light emissionwavelength. Therefore, providing the second mixed layer 216 makes itpossible to prevent light extraction efficiency from varying for eachcolor (i.e., for each wavelength) of light emission. The metal oxide ispreferably contained in the second mixed layer 216 so that a mass ratioof the metal oxide with respect to the aromatic hydrocarbon (=metaloxide/aromatic hydrocarbon) is 0.5 to 2 or a molar ratio thereof is 1 to4. Further, aromatic hydrocarbons have generally a property of beingeasily crystallized; however, when an aromatic hydrocarbon is mixed witha metal oxide like this embodiment mode, the aromatic hydrocarbon is noteasily crystallized. Furthermore, molybdenum oxide is particularlycrystallized easily among metal oxides; however, when molybdenum oxideis mixed with an aromatic hydrocarbon like this embodiment mode, themolybdenum oxide is not easily crystallized. By mixing the aromatichydrocarbon and the metal oxide in this manner, the aromatic hydrocarbonand the metal oxide mutually hinder crystallization, in consequence, alayer which is not easily crystallized can be obtained.

Note that the light emitting element having the hole injecting layer211, the hole transporting layer 212, the electron transporting layer114, and the like in addition to the light emitting layer 213, the firstmixed layer 215, and the second mixed layer 216 is shown in thisembodiment mode; however, a mode of the light emitting element is notnecessarily limited thereto. For example, as shown in FIG. 5, a layer217 containing an aromatic hydrocarbon and a metal oxide, which is muchthe same as the first mixed layer 215, or the like may be provided assubstitute for the hole injecting layer 211. Providing the layer 217containing an aromatic hydrocarbon and a metal oxide makes it possibleto operate the light emitting element favorably even when the firstelectrode 201 is formed using a substance having a low work functionsuch as aluminum and magnesium. Further, as shown in FIG. 6, a holeblocking layer 218 may be provided between the electron transportinglayer 214 and the light emitting layer 213. The hole blocking layer 218is the same as the hole blocking layer 117 described in Embodiment Mode1, and will not be further described in this embodiment mode.

Note that an operator of the present invention may appropriately selectwhether or not the hole injecting layer 211, the hole transporting layer212, and the electron transporting layer 214 are provided. For example,in a case where a defect such as light quenching due to metal is notcaused without providing the hole transporting layer 212, the electrontransporting layer 214, and the like, or in a case where injection ofholes can be preferably performed from the electrode without providingthe hole injecting layer 211, these layers are not necessarily requiredto be provided.

By forming the light emitting element having the second mixed layer 216containing an aromatic hydrocarbon and a metal oxide, defects due tocrystallization of a layer between a pair of electrodes can be reduced.More specifically, a short-circuiting which is caused as a result ofgeneration of concavity and convexity due to the crystallization, can bereduced as compared with a light emitting element having a layer onlycontaining an aromatic hydrocarbon or a metal oxide. Further, since thesecond mixed layer 216 can generate holes, by providing the second mixedlayer 216 containing an aromatic hydrocarbon and a metal oxide, a lightemitting element having less variations in driving voltage, which isdependent on the thickness of the second mixed layer 216, can beobtained. As a consequence, a distance between the light emitting layer213 and the second electrode 202 can be easily adjusted by changing thethickness of the second mixed layer 216. That is, the length of a lightpath through which emitted light passes (light path length), can beeasily adjusted so that light emission can be efficiently extracted toan external portion or color purity of light emission extracted to anexternal portion is improved. Further, by increasing the thickness ofthe second mixed layer 216, the concavity and convexity on the surfaceof the second electrode 202 can be reduced, thereby reducingshort-circuiting between the pair of electrodes.

When the layer containing an aromatic hydrocarbon and a metal oxide isalso provided at the side of the first electrode 201 in place of thehole injecting layer 211, a distance between the first electrode 201 andthe light emitting layer 213 can be easily adjusted. Moreover, concavityand convexity generated on the surface of the first electrode 201 can bereduced, making it possible to reduce short-circuiting between the pairof electrodes.

The light emitting element described above may be formed by using amethod by which the first electrode 201 is first formed, and afterforming respective layers such as the light emitting layer 213, thesecond electrode 202 is formed. Alternatively, the light emitting layermay be formed by using a method by which the second electrode 202 isfirst formed, and after forming respective layers such as the lightemitting layer 213, the first electrode 201 is formed. In each method,by forming a layer containing an aromatic hydrocarbon and a metal oxideafter the formation of the light emitting layer 213, even when theelectrode (either the first electrode 201 or the second electrode 202)is formed by sputtering, the layer containing an aromatic hydrocarbonand a metal oxide serves as a protection layer. Therefore, it ispossible to form a favorable light emitting element, in which damage dueto sputtering of a layer formed using an organic compound such as thelight emitting layer 213 is not easily generated.

Embodiment Mode 3

One mode of a light emitting element of the present invention will bedescribed with reference to FIG. 7. FIG. 7 shows a light emittingelement having a plurality of light emitting layers, i.e., a first lightemitting layer 413 a, a second light emitting layer 413 b, and a thirdlight emitting layer 413 c between a first electrode 401 and a secondelectrode 402. This light emitting element has a first mixed layer 421 aand a second mixed layer 422 a between the first light emitting layer413 a and the second light emitting layer 413 b, and a first mixed layer421 b and a second mixed layer 422 b between the second light emittinglayer 413 b and the third light emitting layer 413 c. Each of the firstmixed layers 421 a and 421 b contains at least one substance selectedfrom the group consisting of an alkali metal, an alkali earth metal, analkali metal oxide, an alkali earth metal oxide, an alkali metalfluoride, and alkali earth metal fluoride; and a substance having anelectron transporting property. The second mixed layers 422 a and 422 beach contain an aromatic hydrocarbon and a metal oxide. Further, thefirst mixed layer 421 a is provided to be closer to the first electrode401 than the second mixed layer 422 a, whereas the first mixed layer 421b is provided to be closer to the first electrode 401 than the secondmixed layer 422 b. A hole transporting layer 412 a is provided betweenthe first electrode 401 and the first light emitting layer 413 a, a holetransporting layer 412 b is provided between the second mixed layer 422a and the second light emitting layer 413 b, and a hole transportinglayer 412 c is provide between the second mixed layer 422 b and thethird light emitting layer 413 c. Further, an electron transportinglayer 414 a is provided between the first light emitting layer 413 a andthe first mixed layer 421 a, an electron transporting layer 414 b isprovided between the second light emitting layer 413 b and the firstmixed layer 421 b, and an electron transporting layer 414 c is providedbetween the third light emitting layer 413 c and the second electrode402. Furthermore, a hole injecting layer 411 is provided between thefirst electrode 401 and the hole transporting layer 412 a, and anelectron injecting layer 415 is provided between the second electrode402 and the electron transporting layer 414 c. Each of the first lightemitting layer 413 a, the second light emitting layer 413 b, and thethird light emitting layer 413 c contains a light emitting substance.When voltage is applied to each of the first electrode 401 and thesecond electrode 402 so that potential of the first electrode 401 ishigher than that of the second electrode 402, holes and electrons arerecombined in each of the light emitting layers, and the light emittingsubstance contained in each of the light emitting layers is excited. Theexcited light emitting substance emits light upon returning to a groundstate. Further, the light emitting substances contained in each of thelight emitting layers may be the same or different from one another.

The first electrode 401 is preferably formed using a substance having ahigh work function such as indium tin oxide, indium tin oxide containingsilicon oxide, indium oxide containing 2 to 20 wt % zinc oxide, gold(Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr),molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), ortantalum nitride. Note that when a layer containing an aromatichydrocarbon and a metal oxide is provided as substitute for the holeinjecting layer 411, the first electrode 401 can be formed by using asubstance having a low work function such as aluminum and magnesium.

The second electrode 402 is preferably formed using a substance having alow work function such as aluminum and magnesium. When a layergenerating electrons is provided between the second electrode 402 andthe third light emitting layer 413 c; however, a substance having a highwork function such as indium tin oxide, indium tin oxide containingsilicon oxide, indium oxide containing 2 to 20 wt % zinc oxide, gold(Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr),molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), ortantalum nitride; and the like can also be used. Therefore, the secondelectrode 402 may be formed by appropriately selecting either asubstance having a high work function or a substance having a low workfunction in accordance with a characteristic of a layer provided betweenthe second electrode 402 and the third light emitting layer 413 c.

In the light emitting element of this embodiment mode, the first mixedlayers 421 a and 421 b are much the same as the first mixed layer 215described in Embodiment Mode 2. Further, the second mixed layers 422 aand 422 b are much the same as the second mixed layer 216 described inEmbodiment Mode 2. Furthermore, the first light emitting layer 413 a,the second light emitting layer 413 b, and the third light emittinglayer 413 c are much the same as the light emitting layer 213 describedin Embodiment Mode 2. Moreover, the hole injecting layer 411, the holetransporting layers 412 a, 412 b, and 412 c, the electron transportinglayers 414 a, 414 b, and 414 c, and the electron injecting layer 415 arealso much the same as the respective layers denoted by the same names inEmbodiment Mode 1 or 2.

By forming the light emitting element having the second mixed layers 422a and 422 b each containing an aromatic hydrocarbon and a metal oxide asdescribed above, defects due to crystallization of a layer between thepair of electrodes can be reduced. More specifically, a short-circuitingwhich is caused as a result of generation of concavity and convexity dueto the crystallization can be reduced as compared with a light emittingelement having a layer only containing an aromatic hydrocarbon or ametal oxide. Further, by providing the second mixed layers 422 a and 422b each containing an aromatic hydrocarbon and a metal oxide, it ispossible to obtain a light emitting element in which damage due tosputtering of a layer formed using an organic compound such as a lightemitting layer is not easily generated as compared with a light emittingelement having a structure in which a layer formed by sputtering like alayer made from indium tin oxide is provided between respective lightemitting layers.

Embodiment Mode 4

One mode of a light emitting element of the present invention will bedescribed with reference to FIG. 8.

FIG. 8 shows a light emitting element having a light emitting layer 3113between a first electrode 3101 and a second electrode 3102. In the lightemitting element shown in FIG. 8, a mixed layer 3111 is provided betweenthe light emitting layer 3113 and the first electrode 3101. Further, ahole transporting layer 3112 is provided between the light emittinglayer 3113 and the mixed layer 3111, and an electron transporting layer3114 and an electron injecting layer 3115 are provided between the lightemitting layer 3113 and the second electrode 3102. In this lightemitting element, when voltage is applied to both the first electrode3101 and the second electrode 3102 so that potential of the firstelectrode 3101 is higher than that of the second electrode 3102, holesare injected to the light emitting layer 3113 from the first electrode3101 side whereas electrons are injected to the light emitting layer3113 from the second electrode 3102 side. Then, the holes and electronsinjected to the light emitting layer 3113 are recombined. A lightemitting substance is contained in the light emitting layer 3113, andthe light emitting substance is excited by excitation energy generatedby the recombination. The light emitting substance in the excited stateemits light upon returning to a ground state.

The first electrode 3101, the second electrode 3102, and each layerprovided between the first electrode 3101 and the second electrode 3102will be described in detail below.

A substance used for forming the first electrode 3101 is notparticularly limited. A substance having a low work function such asaluminum and magnesium can be used in addition to a substance having ahigh work function such as indium tin oxide, indium tin oxide containingsilicon oxide, indium oxide containing 2 to 20 wt % zinc oxide, gold(Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr),molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), ortantalum nitride. This is because holes are generated from the mixedlayer 3111 when applying voltage to the light emitting element of thepresent invention.

The second electrode 3102 is preferably formed using a substance havinga low work function such as aluminum and magnesium. When a layergenerating electrons is provided between the second electrode 3102 andthe light emitting layer 3113, however, a substance having a high workfunction such as indium tin oxide, indium tin oxide containing siliconoxide, indium oxide containing 2 to 20 wt % zinc oxide, gold (Au),platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), or tantalumnitride; and the like can also be used. Therefore, the second electrode3102 may be formed by appropriately selecting either a substance havinga high work function or a substance having a low work function inaccordance with a characteristic of a layer provided between the secondelectrode 3102 and the light emitting layer 3113.

Note that one or both the first electrode 3101 and the second electrode3102 is/are preferably formed so that one or both of the electrodes cantransmit light.

The mixed layer 3111 contains an aromatic hydrocarbon and a metal oxide.The aromatic hydrocarbon is not particularly limited; however, anaromatic hydrocarbon having hole mobility of 1×10⁻⁶ cm²/Vs or more (morepreferably, 1× 10⁻⁶ to 1× 10^(0 cm) ²/Vs) is preferably used. As such anaromatic hydrocarbon having a favorable hole transporting property,which can be used for forming the mixed layer 3111, for example,aromatic hydrocarbons having 14 to 60 carbon atoms and containing ananthracene skeleton such as 2-tert-butyl-9,10-di(2-naphthyl) anthracene(abbreviation: t-BuDNA); 910-di(naphthalen-1-yl)-2-tert-butylanthracene; anthracene;9,10-diphenylanthracene (abbreviation: DPAnth);9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA);9,10-di(naphthalen-2-yl)anthracene (abbreviation: DNA);2-tert-butylanthracene (abbreviation: t-BuAnth);9,10-di(4-methylnaphthalen-1-yl)anthracene (abbreviation: DMNA);9,10-bis[2-(naphthalen-1-yl)phenyl]anthracene;2,3,6,7-tetramethyl-9,10-di(naphthalen-1-yl)anthracene;2,3,6,7-tetramethyl-9,10-di(naphthalen-2-yl)anthracene; bianthryl;10,10′-di(2-phenylphenyl)-9,9′-bianthryl;10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9-9′-bianthryl; and9,10-di(4-phenylphenyl)-2-tert-butylanthracene (abbreviation: t-BuDBA),can be given. Among aromatic hydrocarbons having 14 to 60carbon atoms,in particular, an aromatic hydrocarbon having 26 to 60 carbon atoms ispreferably used. More preferably, an aromatic hydrocarbon having 34 to60 carbon atoms is used. When using the aromatic hydrocarbon having 26to 60 carbon atoms, a heat resistance property of the mixed layer isimproved. In particular, when using the aromatic hydrocarbon having 34to 60 carbon atoms, the mixed layer 3111 showing an extremely favorableheat resistance property can be obtained. Improvement of the heatresistance property of the mixed layer 3111 is extremely effective inmanufacturing a light emitting element in which deterioration inelectric characteristics due to Joule heat generated by current, such asincrease in resistance is inhibited. As the metal oxide, a metal oxideshowing an electron accepting property to an aromatic hydrocarbon ispreferable. As such a metal oxide, for example, molybdenum oxide,vanadium oxide, ruthenium oxide, rhenium oxide, and the like can begiven. In addition, metal oxides such as titanium oxide, chromium oxide,zirconium oxide, hafnium oxide, tantalum oxide, tungsten oxide, orsilver oxide can also be used. By mixing a metal oxide and an aromatichydrocarbon as mentioned above, charge-transfer can be generated so thata charge-transfer complex can be generated. The mixed layer 3111 inwhich a charge-transfer complex is generated contains unpairedelectrons. The mixed layer 3111 in which a charge-transfer complex isgenerated contains unpaired electrons. Further, as compared with a mixedfilm in which divanadium pentoxide and a-NPD are mixed disclosed in arelated art as described above (Japanese Patent Application Laid-Open

No. 2005-123095) and a mixed film in which dirhenium heptaoxide andα-NPD are mixed, the mixed layer 3111 containing a metal oxide and anaromatic hydrocarbon shows less variations in absorbance, which isdependent on an absorption wavelength in a visible light region which isa region of 450 to 650 nm (i.e., less variations in transmittance in avisible light region), and therefore, the amount of light emission losscaused by absorption of light emission in the mixed layer 3111 is lessdependent on a light emission wavelength. Therefore, providing the mixedlayer 3111 makes it possible to prevent light extraction efficiency fromvarying for each color (i.e., for each wavelength) of light emission.The metal oxide is preferably contained in the mixed layer 3111 so thata mass ratio of the metal oxide with respect to the aromatic hydrocarbon(=metal oxide/aromatic hydrocarbon) is 0.5 to 2 or a molar ratio thereofis 1 to 4. When the mixed layer 3111 contains a metal oxide and anaromatic hydrocarbon with such a mixture ratio, the transmittance oflight in a spectrum of 450 to 650 nm of the mixed layer 3111 can be setto be 80% or more, and specifically, 80 to 100%. Such the transmittanceallows a light emitting element to extract light emission moreefficiently. Further, aromatic hydrocarbons have generally a property ofbeing easily crystallized; however, when an aromatic hydrocarbon ismixed with a metal oxide like this embodiment mode, the aromatichydrocarbon is not easily crystallized. Furthermore, molybdenum oxide isparticularly crystallized easily among metal oxides; however, whenmolybdenum oxide is mixed with an aromatic hydrocarbon like thisembodiment mode, the molybdenum oxide is not easily crystallized. Bymixing an aromatic hydrocarbon and a metal oxide in this manner, thearomatic hydrocarbon and the metal oxide mutually hindercrystallization, making it possible to form a layer which is not easilycrystallized. Moreover, since aromatic hydrocarbons have a high glasstransition temperature, when the mixed layer 3111 containing an aromatichydrocarbon is applied as a mixed layer like this embodiment mode, amixed layer having an excellent heat resistance property along with afunction of favorably injecting holes also to the hole transportinglayer 3112 can be obtained as compared with a hole injecting layerformed using 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA);4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA);4,4′-bis{N-[4-(N,N-di-m-tolylamino)phenyl]-N-phenylaminol}biphenyl(abbreviation: DNTPD); or the like.

The hole transporting layer 3112 has a function of transporting holes.In the light emitting element of this embodiment mode, the holetransporting layer 3112 has a function of transporting holes to thelight emitting layer 3113 from the mixed layer 3111. Providing the holetransporting layer 3112 makes it possible to create a distance betweenthe mixed layer 3111 and the light emitting layer 3113. Consequently, itis possible to prevent quenching of light emission due to metalcontained in the mixed layer 3111. The hole transporting layer 3112 ispreferably formed using a substance having a hole transporting property.In particular, the hole transporting layer 3112 is preferably formedusing a substance having a hole transporting property with hole mobilityof 1×10⁻⁶ cm²/Vs or more (more preferably, 1×10⁻⁶ to 1×10⁰ cm²/Vs), or abipolar substance. Further, the substance having the hole transportingproperty is a substance having higher hole mobility than electronmobility, and corresponds to a substance in which a ratio of holemobility to electron mobility (i.e., hole mobility/electron mobility) islarger than 100. As specific examples of the substance having the holetransporting property, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB); 4,4′-bis[N-(3-methylphenyl)-N-phenylamino]biphenyl(abbreviation: TPD); 1,3,5-tris[N,N-di(m-tolyl)amino]benzene(abbreviation: m-MTDAB); 4,4′,4″-tris(N-carbazolyl)triphenylamine(abbreviation: TCTA); and the like can be given. Further, the bipolarsubstance is a substance in which a ratio of mobility of one carrier tomobility of the other carrier is 100 or less, and preferably, 10 orless. As the bipolar substance, for example,2,3-bis(4-diphenylaminophenyl)quinoxaline (abbreviation: TPAQn);2,3-bis{4-[N-(1-naphthyl)-N-phenylamino]phenyl}-dibenzo[f,h]quinoxaline(abbreviation: NPADiBzQn); and the like can be given. Among bipolarsubstances, in particular, a bipolar substance having mobility of holesand electrons of 1×10⁻⁶ cm²/Vs or more (more preferably, 1×10⁻⁶ to 1×10⁰cm²/Vs) is preferably used.

The light emitting layer 3113 contains a light emitting substance. Thelight emitting substance is a substance having favorable light emittingefficiency which can emit light with a desired wavelength. The lightemitting layer 3113 may be formed using only a light emitting substance.When quenching due to a concentration is caused, the light emittinglayer 3113 is preferably a layer in which a light emitting substance isdispersed in a substance having a larger energy gap than that of thelight emitting substance. By mixing the light emitting substance in thelight emitting layer 3113, quenching of light emission due to aconcentration can be prevented. Here, an energy gap indicates an energygap between a LUMO level and a HOMO level.

The light emitting substance is not particularly limited, and asubstance with favorable light emitting efficiency, which can emit lightwith a desired light emission wavelength, may be used. For example, inorder to obtain red light emission, for example, the followingsubstances showing emission spectrum with peaks in a spectrum of 600 to680 nm can be employed:4-dicyanomethylene-2-isopropyl-6-[2-(1,1,7,7-tetramethyl-9-julolidyl)ethenyl]-4H-pyran(abbreviation: DCJTI);4-dicyanomethylene-2-methyl-6-[2-(1,1,7,7-tetramethyl-9-julolidyl)ethenyl]-4H-pyran(abbreviation: DCJT);4-dicyanomethylene-2-tert-butyl-6-[2-(1,1,7,7-tetramethyl-9-julolidyl)ethenyl]-4H-pyran(abbreviation: DCJTB); periflanthene;2,5-dicyano-1,4-bis[2-(10-methoxy-1,1,7,7-tetramethyl-9-julolidyl)ethenyl]benzene;and the like. In order to obtain green light emission, substancesshowing emission spectrum with peaks in a spectrum of 500 to 550 nm suchas N,N′-dimethylquinacridon (abbreviation: DMQd), coumarin 6, coumarin545T, and tris(8-quinolinolato)aluminum (abbreviation: Alq₃) can beemployed. In order to obtain blue light emission, the followingsubstances showing emission spectrum with peaks in a spectrum of 420 to500 nm can be employed: 9,10-bis(2-naphthyl)-tert-butylanthracene(abbreviation: t-BuDNA); 9,9′-bianthryl; 9,10-diphenylanthracene(abbreviation: DPA); 9,10-bis(2-naphthyl)anthracene (abbreviation: DNA);bis(2-methyl-8-quinolinolato)-4-phenylphenolato-gallium (abbreviation:BGaq); bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum(abbreviation: BAlq); and the like. In addition to the above mentionedsubstances, which emit fluorescence, the following substances, whichemit phosphorescence, can be used as the light emitting substance:bis[2-(3,5-bis(trifluoromethyl)phenyl)pyridinato-N,C²′]iridium(III)picolinato(abbreviation: Ir(CF₃ppy)₂(pic));bis[2-(4,6-difluorophenyl)pyridinato-N,C²′]iridium(III)acetylacetonato(abbreviation: FIr(acac));bis[2-(4,6-difluorophenyl)pyridinato-N,C²′]iridium(III)picolinato(abbreviation: FIr(pic)); tris(2-phenylpyridinato-N,C²′)iridium(abbreviation: Ir(ppy)₃); and the like.

A substance used for dispersing a light emitting substance, which iscontained in the light emitting layer 3113 along with the light emittingsubstance (also referred to as a host) is not particularly limited, andmay be appropriately selected in consideration for an energy gap and thelike of a substance used for the light emitting substance. For example,an anthracene derivative such as9,10-di(2-naphthyl)-2-tert-butylanthracene (abbreviation: t-BuDNA); acarbazole derivative such as 4,4′-bis(N-carbazolyl)biphenyl(abbreviation: CBP); a quinoxaline derivative such as2,3-bis(4-diphenylaminophenyl)quinoxaline (abbreviation: TPAQn), and2,3-bis{4-[N-(1-naphthyl)-N-phenylamino]phenyl}-dibenzo[f,h]quinoxaline(abbreviation: NPADiBzQn); a metal complex such asbis[2-(2-hydroxyphenyl)pyridinato]zinc (abbreviation: Znpp₂), andbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: ZnBOX); and thelike can be used. Note that a light emitting substance is generallyreferred to as a guest with respect to the host.

The electron transporting layer 3114 has a function of transportingelectrons. In the light emitting element of this embodiment mode, theelectron transporting layer 3114 has a function of transportingelectrons injected from the second electrode 3102 side to the lightemitting layer 3113. Providing the electron transporting layer 3114makes it possible to create a distance between the second electrode 3102and the light emitting layer 3113. Consequently, quenching of lightemission due to metal contained in the second electrode 3102 can beprevented. The electron transporting layer is preferably formed using asubstance having an electron transporting property. In particular, theelectron transporting layer is preferably formed using a substancehaving an electron transporting property with electron mobility of1×10⁻⁶ cm²/Vs or more (more preferably, 1×10⁻⁶ to 1×10⁰ cm²/Vs), or abipolar substance. Further, the substance having the electrontransporting property is a substance having higher electron mobilitythan hole mobility, and corresponds to a substance in which a ratio ofelectron mobility to hole mobility (i.e., electron mobility/holemobility) is larger than 100. As specific examples of the substancehaving the electron transporting property, a metal complex such astris(8-quinolinolato)aluminum (abbreviation: Alq₃),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]-quinolinato)beryllium (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)-4-phenylphenolato-aluminum (abbreviation:BAlq), bis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation:Zn(BOX)₂), and bis[2-(2-hydroxyphenyl)benzothiazolato]zinc(abbreviation: Zn(BTZ)₂);2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD); 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene(abbreviation: OXD-7); TAZ; p-EtTAZ; BPhen; BCP;4,4′-bis(5-methylbenzoxazolyl-2-yl)stilbene (abbreviation: BzOS); andthe like can be given. Further, the bipolar substance is the same asdescribed above. Note that the electron transporting layer 3114 and thehole transporting layer 3112 may be formed using the same bipolarsubstance.

The electron injecting layer 3115 has a function of helping injection ofelectrons to the electron transporting layer 3114 from the secondelectrode 3102. By providing the electron injecting layer 3115,difference in electron affinity between the second electrode 3102 andthe electron transporting layer 3114 can be reduced, thereby injectingelectrons easily. The electron injecting layer 3115 is preferably formedusing a substance whose electron affinity is larger than that of asubstance of the electron transporting layer 3114 and smaller than thatof a substance of the second electrode 3102, or a substance whose energyband is curved when it is provided as a thin film with a thickness ofabout 1 to 2 nm between the electron transporting layer 3114 and thesecond electrode 3102. As specific examples of substances, which can beused for forming the electron injecting layer 3115, the followinginorganic materials can be given: alkali metals such as lithium (Li),and the like; alkali earth metals such as magnesium (Mg), and the like;alkali metal fluorides such as cesium fluoride (CsF), and the like;alkali earth metal fluorides such as calcium fluoride (CaF₂), and thelike; alkali metal oxides such as lithium oxide (Li₂O), sodium oxide(Na₂O), potassium oxide (K₂O), and the like; alkali earth metal oxidessuch as calcium oxide (CaO), magnesium oxide (MgO), and the like. Whenthese substances are formed as thin films, their energy bands can becurved and the injection of electrons can be carried out easily, andtherefore, these substances are preferable. In addition to the inorganicmaterials, organic materials, which can be used for forming the electrontransporting layer 3114 such as bathophenanthroline (abbreviation:BPhen), bathocuproin (abbreviation: BCP),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), and3-(4-tert-butylphenyl)-4-phenyl-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: TAZ), can be used to form the electron injecting layer3115 by selecting a substance having higher electron affinity than thatof a substance used for forming the electron transporting layer 3114from among them. That is, the electron injecting layer 3115 can beformed by selecting a substance having electron affinity which isrelatively higher than that of the electron transporting layer 3114 fromsubstances having electron transporting properties. Further, in a caseof providing the electron injecting layer 3115, the second electrode3102 is preferably formed using a substance having a low work functionsuch as aluminum.

In the light emitting element as described above, a substance having anelectron transporting property used for the electron transporting layer3114 and an aromatic hydrocarbon contained in the mixed layer 3111 arerespectively selected so that when mobility of the substance having theelectron transporting property used for the electron transporting layer3114 is compared with mobility of the aromatic hydrocarbon contained inthe mixed layer 3111, a mobility ratio of one of the substance havingthe electron transporting property and the aromatic hydrocarbon to theother is preferably set to be 1,000 or less. By selecting the respectivesubstances, recombination efficiency in the light emitting layer can beimproved.

Note that the light emitting element having the hole transporting layer3112, the electron transporting layer 3114, the electron injecting layer3115, and the like in addition to the mixed layer 3111 and the lightemitting layer 3113 is shown in this embodiment mode; however, a mode ofthe light emitting element is not necessarily limited thereto. Forexample, as shown in FIG. 9, an electron generating layer 3116 and thelike may be provided as substitute for the electron injecting layer3115. The electron generating layer 3116 generates electrons, and can beformed by mixing at least one substance of one or more of a substancehaving an electron transporting property or a bipolar substance and asubstance showing an electron donating property with respect to theabove substance. In this case, a substance having electron mobility of1×10⁻⁶ cm²/Vs or more (more preferably, 1×10⁻⁶ to 1×10⁰ cm²/Vs) ispreferably used among the substances having the electron transportingproperties and the bipolar substances. As the substance having theelectron transporting property and the bipolar substance, the abovementioned substances can be used, respectively. Further, as thesubstance showing the electron donating property, at least one substanceof selected from the group consisting of alkali metals and alkali earthmetals, such as lithium (Li), calcium (Ca), sodium (Na), potassium (K),magnesium (Mg), and the like can be used. Furthermore, at least onesubstance selected from the group consisting of alkali metal oxides,alkali earth metal oxides, alkali metal fluorides, alkali earth metalfluorides, and the like such as lithium oxide (Li₂O), calcium oxide(CaO), sodium oxide (Na₂O), potassium oxide (K₂O), magnesium oxide(MgO), lithium fluoride (LiF), cesium fluoride (CsF), and the like canbe used as the substance showing the electron donating property.

Further, as shown in FIG. 10, a hole blocking layer 3117 may be providedbetween the light emitting layer 3113 and the electron transportinglayer 3114. Providing the hole blocking layer 3117 makes it possible toprevent holes from flowing toward the second electrode 3102 through thelight emitting layer 3113, thereby improving recombination efficiency ofcarriers. In addition, excitation energy generated in the light emittinglayer 3113 can be prevented from transferring to other layer such as theelectron transporting layer 3114. The hole blocking layer 3117 can beformed by selecting, in particular, a substance having larger ionizationpotential and larger excitation energy than those of a substance usedfor forming the light emitting layer 3113 from among substances, whichcan be used for forming the electron transporting layer 3114, such asBAlq, OXD-7, TAZ, or BPhen. That is, the hole blocking layer 3117 may beformed by selecting a substance so that ionization potential of the holeblocking layer 3117 is relatively larger than that of the electrontransporting layer 3114. Similarly, a layer for blocking electrons fromflowing toward the second electrode 3102 through the light emittinglayer 3113 may be provided between the light emitting layer 3113 and thehole transporting layer 3112.

Note that an operator of the present invention may appropriately selectwhether or not the electron injecting layer 3115, the electrontransporting layer 3114, and the hole transporting layer 3112 areprovided. For example, in a case where a defect such as an opticalquenching due to metal is not caused without providing the holetransporting layer 3112, the electron transporting layer 3114, and thelike, or in a case where injection of electrons can be preferablyperformed from the electrode without providing the electron injectinglayer 3115, these layers are not necessarily required to be provided.

By forming the light emitting layer having the mixed layer 3111containing an aromatic hydrocarbon and a metal oxide, a defect due tocrystallization of a layer between a pair of electrodes can be reduced.More specifically, a short-circuiting which is caused as a result ofgeneration of concavity and convexity due to the crystallization, can bereduced as compared with a light emitting element having a layer onlycontaining an aromatic hydrocarbon or a metal oxide. Further, since themixed layer 3111 can generate holes, by providing the mixed layer 3111containing an aromatic hydrocarbon and a metal oxide, a light emittingelement having less variations in driving voltage, which is dependent onthe thickness of the mixed layer 3111, can be obtained. As aconsequence, a distance between the light emitting layer 3113 and thefirst electrode 3101 can be easily adjusted by changing the thickness ofthe mixed layer 3111. That is, the length of a light path through whichemitted light passes (light path length), can be easily adjusted so thatlight emission can be efficiently extracted to an external portion orcolor purity of light extracted to an external portion is improved.Further, by increasing the thickness of the mixed layer 3111, theconcavity and convexity over the surface of the first electrode 3101 canbe reduced, thereby reducing short-circuiting between the pair ofelectrodes.

The light emitting element described above may be formed by using amethod by which the mixed layer 3111, the hole transporting layer 3112,the light emitting layer 3113, the electron transporting layer 3114, theelectron injecting layer 3115, and the like are stacked over the firstelectrode 3101 in this order. Alternatively, the light emitting layermay be formed by using a method by which the electron injecting layer3115, the electron transporting layer 3114, the light emitting layer3113, the hole transporting layer 3112, and the mixed layer 3111 arestacked over the second electrode 3102 in this order. In a case wherethe mixed layer 3111 is formed after the formation of the light emittinglayer 3113 like the latter method, even when the first electrode 3101 isformed by sputtering, the mixed layer 3111 serves as a protection layer,and hence, it is possible to form a favorable light emitting element inwhich damage due to sputtering of a layer formed using an organiccompound such as the light emitting layer 3113 is not easily generated.

Embodiment Mode 5

One mode of a light emitting element of the present invention will bedescribed with reference to FIG. 11

FIG. 11 shows a light emitting element having a light emitting layer3213, a first mixed layer 3215, and a second mixed layer 3216 between afirst electrode 3201 and a second electrode 3202, in which the lightemitting layer 3213 is provided to be closer to the second electrode3201 than the first mixed layer 3215 and the second mixed layer 3216 isprovided to be closer to the second electrode 3202 than the first mixedlayer 3215. In this light emitting element, a hole injecting layer 3211and a hole transporting layer 3212 are provided between the lightemitting layer 3213 and the first electrode 3201, and an electrontransporting layer 3214 is provided between the light emitting layer3213 and the first mixed layer 3215. The first mixed layer 3215 containsat least one substance selected from the group consisting of an alkalimetal, an alkali earth metal, an alkali metal oxide, an alkali earthmetal oxide, an alkali metal fluoride, and an alkali earth metalfluoride, and a substance having an electron transporting property. Thesecond mixed layer 3216 contains an aromatic hydrocarbon and a metaloxide. The light emitting layer 3213 contains a light emittingsubstance. When voltage is applied to each of the first electrode 3201and the second electrode 3202 so that potential of the first electrode3201 is higher than that of the second electrode 3202, electrons areinjected to the electron transporting layer 3214 from the first mixedlayer 3215, holes are injected to the second electrode 3202 from thesecond mixed layer 3216, and holes are injected to the hole injectinglayer 3211 from the first electrode 3201. Further, holes injected to thelight emitting layer 3213 from the first electrode 3201 side andelectrons injected to the light emitting layer 3213 from the secondelectrode 3202 side are recombined. The light emitting substancecontained in the light emitting layer 3213 is excited by excitationenergy generated by the recombination. The excited light emittingsubstance emits light upon returning to a ground state.

The first electrode 3201, the second electrode 3202, and each layerprovided between the first electrode 3201 and the second electrode 3202will be described in detail below.

The first electrode 3201 is preferably formed using a substance having ahigh work function such as indium tin oxide, indium tin oxide containingsilicon oxide, indium oxide containing 2 to 20 wt % zinc oxide, gold(Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr),molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), ortantalum nitride.

The second electrode 3202 is preferably formed using a substance havinga low work function such as aluminum and magnesium. When a layergenerating electrons is provided between the second electrode 3202 andthe light emitting layer 3213, however, a substance having a high workfunction such as indium tin oxide, indium tin oxide containing siliconoxide, indium oxide containing 2 to 20 wt % zinc oxide, gold (Au),platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr), molybdenum(Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), or tantalumnitride; and the like can also be used. Therefore, the second electrode3202 may be formed by appropriately selecting either a substance havinga high work function or a substance having a low work function inaccordance with a characteristic of a layer provided between the secondelectrode 3202 and the light emitting layer 3213.

Note that one or both the first electrode 3201 and the second electrode3202 is/are preferably formed so that one or both of the electrodes cantransmit light.

The hole injecting layer 3211 of this embodiment mode has a function ofhelping injection of holes to the hole transporting layer 3212 from thefirst electrode 3201. Providing the hole injecting layer 3211 makes itpossible to reduce difference in ionization potential between the firstelectrode 3201 and the hole transporting layer 3212 so that holes areeasily injected. The hole injecting layer 3211 is preferably formedusing a substance whose ionization potential is smaller than that of asubstance used for forming the hole transporting layer 3223 and largerthan that of a substance used for forming the first electrode 3201. Asspecific examples of substances which can be used for forming the holeinjecting layer 3211, a low molecular material such as phthalocyanine(abbreviation: H₂Pc) or copper phthalocyanine (abbreviation: CuPc), ahigh molecular material such as apoly(ethylenedioxythiophene)/poly(styrene sulfonate) solution(PEDOT/PSS), and the like can be given.

The hole transporting layer 3212 has a function of transporting holes.In the light emitting element of this embodiment mode, the holetransporting layer 3212 has a function of transporting holes to thelight emitting layer 3213 from the hole injecting layer 3211. Providingthe hole transporting layer 3212 makes it possible to create a distancebetween the first electrode 3201 and the light emitting layer 3213.Consequently, it is possible to prevent quenching of light emission dueto metal contained in the first electrode 3201. The hole transportinglayer 3212 is preferably formed using a substance having a holetransporting property. In particular, the hole transporting layer 3212is preferably formed using a substance having a hole transportingproperty with hole mobility of 1×10⁻⁶ cm²/Vs or more or a bipolarsubstance. Further, the substance having the hole transporting propertyand the bipolar substance are already described in Embodiment Mode 4,and will not be further described in this embodiment mode.

The light emitting layer 3213 contains a light emitting substance. Thelight emitting layer 3213 may be formed using only a light emittingsubstance. When quenching due to a concentration is caused, the lightemitting layer 3213 is preferably a layer in which a light emittingsubstance is dispersed in a substance having a larger energy gap thanthat of the light emitting substance. By mixing the light emittingsubstance in the light emitting layer 3213, quenching of light emissiondue to a concentration can be prevented. Here, an energy gap indicatesan energy gap between a LUMO level and a HOMO level. The light emittingsubstance and the substance used for dispersing the light emittingsubstance are already described in Embodiment Mode 4, and will not befurther described in this embodiment mode.

The electron transporting layer 3214 has a function of transportingelectrons. In the light emitting element of this embodiment mode, theelectron transporting layer 3214 has a function of transportingelectrons injected from the first mixed layer 3215 to the light emittinglayer 3213. Providing the electron transporting layer 3214 makes itpossible to create a distance between the second mixed layer 3216 andthe light emitting layer 3213. Consequently, quenching of light emissiondue to metal contained in the second mixed layer 3216 (when metal iscontained in the first mixed layer 3215, quenching of light emission dueto the metal) can be prevented. The electron transporting layer 3214 ispreferably formed using a substance having an electron transportingproperty. In particular, the electron transporting layer is preferablyformed using a substance having an electron transporting property withelectron mobility of 1×10⁻⁶ cm²/Vs or more (more preferably, 1×10⁻⁶ to1×10⁰ cm²/Vs), or a bipolar substance. Further, the substance having theelectron transporting property and the bipolar substance are alreadydescribed in Embodiment Mode 4, and will no be further described in thisembodiment mode.

The first mixed layer 3215 generates electrons. The first mixed layer3215 can be formed by using a mixture of at least one substance of oneor more of a substance having an electron transporting property or abipolar substance and a substance showing an electron donating propertywith respect to the substance. In this case, a substance having electronmobility of 1×10⁻⁶ cm²/Vs or more is preferably used among the substancehaving the electron transporting property and the bipolar substance.Further, the substance having the electron transporting property and thebipolar substance are already described in Embodiment Mode 4, and willnot be further described in this embodiment mode. Furthermore, thesubstance showing the electron donating property with respect to thesubstance having the electron transporting property and the bipolarsubstance is also described in Embodiment Mode 4, and will not befurther described in this embodiment mode.

The second mixed layer 3216 contains an aromatic hydrocarbon and a metaloxide. The aromatic hydrocarbon is not particularly limited; however, anaromatic hydrocarbon having hole mobility of 1×10⁻⁶ cm²/Vs or more (morepreferably, 1×10⁻⁶ cm²/Vs to 1×10⁰ cm²/Vs) is preferably used. As thearomatic hydrocarbon having a favorable hole transporting property,which can be used for forming the second mixed layer 3216, for example,aromatic hydrocarbons having 14 to 60 carbon atoms and containing ananthracene skeleton such as 2-tert-butyl-9,10-di(2-naphthyl)anthracene;9,10-di(naphthalen-1-yl)-2-tert-butyl anthracene; anthracene; 9,10-diphenylanthracene; 9,10-bis(3,5-diphenylphenyl)anthracene;9,10-di(naphthalen-2-yl)anthracene; 2-tert-butylanthracene;9,10-di(4-methylnaphthalen-1-yl)anthracene;9,10-bis[2-(naphthalen-1-yl)phenyl]anthracene;2,3,6,7-tetramethyl-9,10-di(naphthalen-1-yl)anthracene;2,3,6,7-tetramethyl-9,10-di(naphthalen-2-yl)anthracene; bianthryl;10,10′-di(2-phenylphenyl)-9,9′-bianthryl;10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl;9,10-di(4-phenylphenyl)-2-tert-butylanthracene; and the like can begiven. Among aromatic hydrocarbons having 14 to 60 carbon atoms, inparticular, an aromatic hydrocarbon having 26 to 60 carbon atoms ispreferably used. More preferably, an aromatic hydrocarbon having 34 to60 carbon atoms is used. When using the aromatic hydrocarbon having 26to 60 carbon atoms, a heat resistance property of the mixed layer isimproved. In particular, when using the aromatic hydrocarbon showing 34to 60 carbon atoms, the second mixed layer 3216 having an extremelyfavorable heat resistance property can be obtained. Improvement of theheat resistance property of the second mixed layer 3216 is extremelyeffective in manufacturing a light emitting element in whichdeterioration in electric characteristics due to Joule heat generated bycurrent such as increase in resistance is inhibited. As the metal oxide,a metal oxide showing an electron accepting property to an aromatichydrocarbon is preferable. As such a metal oxide, for example,molybdenum oxide, vanadium oxide, ruthenium oxide, rhenium oxide, andthe like can be given. In addition, metal oxides such as titanium oxide,chromium oxide, zirconium oxide, hafnium oxide, tantalum oxide, tungstenoxide, or silver oxide can also be used. By mixing a metal oxide and anaromatic hydrocarbon as mentioned above, charge-transfer can begenerated. The second mixed layer 3216 containing a metal oxide and anaromatic hydrocarbon shows extremely less variations in absorbance,which is dependent on an absorption wavelength in a visible light regionwhich is a region of 450 to 650 nm (i.e., less variations intransmittance in a visible light region), and therefore, the amount oflight emission loss caused by absorption of light emission in the secondmixed layer 3216 is less dependent on a light emission wavelength.Accordingly, providing the second mixed layer 3216 makes it possible toprevent light extraction efficiency from varying for each color (i.e.,for each wavelength) of light emission. The metal oxide is preferablycontained in the second mixed layer 3216 so that a mass ratio of themetal oxide with respect to the aromatic hydrocarbon (=metal oxide /aromatic hydrocarbon) is 0.5 to 2 or a molar ratio thereof is 1 to 4.Further, aromatic hydrocarbons generally have a property of being easilycrystallized; however, when an aromatic hydrocarbon is mixed with ametal oxide, the aromatic hydrocarbon is not easily crystallized.Furthermore, molybdenum oxide is particularly crystallized easily amongmetal oxide; however, when molybdenum oxide is mixed with an aromatichydrocarbon like this embodiment mode, the molybdenum oxide is noteasily crystallized. By mixing an aromatic hydrocarbon and a metal oxidein this manner, the aromatic hydrocarbon and the metal oxide mutuallyhinder crystallization, making it possible to form a layer which is noteasily crystallized.

Note that the light emitting element having the hole injecting layer3211, the hole transporting layer 3212, the electron transporting layer3214, and the like in addition to the light emitting layer 3213, thefirst mixed layer 3215, and the second mixed layer 3216 is shown in thisembodiment mode; however, a mode of the light emitting element is notnecessarily limited thereto. For example, as shown in FIG. 12, a layer3217 containing an aromatic hydrocarbon and a metal oxide, or the likemay be provided as substitute for the hole injecting layer 3211.Providing the layer 3217 containing an aromatic hydrocarbon and a metaloxide makes it possible to operate the light emitting element favorablyeven when the first electrode 3201 is formed using a substance having alow work function such as aluminum and magnesium. Further, as shown inFIG. 13, a hole blocking layer 3218 may be provided between the electrontransporting layer 3214 and the light emitting layer 3213. The holeblocking layer 3218 is the same as the hole blocking layer 3117described in Embodiment Mode 4, and will not be further described inthis embodiment mode.

Note that an operator of the present invention may appropriately selectwhether or not the hole injecting layer 3211, the hole transportinglayer 3212, and the electron transporting layer 3214 are provided. Forexample, in a case where a defect such as light quenching due to metalis not caused without providing the hole transporting layer 3212, theelectron transporting layer 3214, and the like, or in a case whereinjection of holes can be preferably performed from the electrodewithout providing the hole injecting layer 3211, these layers are notnecessarily required to be provided.

By forming the light emitting element having the second mixed layer 3216containing an aromatic hydrocarbon and a metal oxide, defects due tocrystallization of a layer between a pair of electrodes can be reduced.More specifically, a short-circuiting which is caused as a result ofgeneration of concavity and convexity due to the crystallization, can bereduced as compared with a light emitting element having a layer onlycontaining an aromatic hydrocarbon or a metal oxide. Further, since thesecond mixed layer 3216 can generate holes, by providing the secondmixed layer 3216 containing an aromatic hydrocarbon and a metal oxide, alight emitting element having less variations in driving voltage, whichis dependent on the thickness of the second mixed layer 3216, can beobtained. As a consequence, a distance between the light emitting layer3213 and the second electrode 3202 can be easily adjusted by changingthe thickness of the second mixed layer 3216. That is, the length of alight path through which emitted light passes (light path length), canbe easily adjusted so that light emission can be efficiently extractedto an external portion or color purity of light extracted to an externalportion is improved. Further, by increasing the thickness of the secondmixed layer 3216, the concavity and convexity on the surface of thesecond electrode 3202 can be reduced, thereby reducing short-circuitingbetween the pair of electrodes.

When the layer containing an aromatic hydrocarbon and a metal oxide isalso provided at the side of the first electrode 3201 in place of thehole injecting layer 3211, a distance between the first electrode 3201and the light emitting layer 3213 can be easily adjusted. Moreover,concavity and convexity generated over the surface of the firstelectrode 3201 can be reduced, making it possible to reduceshort-circuiting between the pair of electrodes.

The light emitting element described above may be formed by using amethod by which the first electrode 3201 is first formed and afterforming respective layers such as the light emitting layer 3213, thesecond electrode 3202 is formed. Alternatively, the light emitting layermay be formed by using a method by which the second electrode 3202 isfirst formed, and after forming respective layers such as the lightemitting layer 3213, the first electrode 3201 is formed. In each method,by forming a layer containing an aromatic hydrocarbon and a metal oxideis formed after the formation of the light emitting layer 3213, evenwhen the electrode (either the first electrode 3201 or the secondelectrode 3202) is formed by sputtering, the layer containing anaromatic hydrocarbon and a metal oxide serves as a protection layer.Therefore, it is possible to form a favorable light emitting element inwhich damage due to sputtering of a layer formed using an organiccompound such as the light emitting layer 3213 is not easily generated.

Embodiment Mode 6

One mode of a light emitting element of the present invention will bedescribed with reference to FIG. 14. FIG. 14 shows a light emittingelement having a plurality of light emitting layers, i.e., a first lightemitting layer 3413 a, a second light emitting layer 3413 b, and a thirdlight emitting layer 3413 c between a first electrode 3401 and a secondelectrode 3402. This light emitting element has a first mixed layer 3421a and a second mixed layer 3422 a between the first light emitting layer3413 a and the second light emitting layer 3413 b and a first mixedlayer 3421 b and a second mixed layer 3422 b between the second lightemitting layer 3413 b and the third light emitting layer 3413 c. Each ofthe first mixed layers 3421 a and 3421 b contains at least one substanceselected from the group consisting of an alkali metal, an alkali earthmetal, an alkali metal oxide, an alkali earth metal oxide, an alkalimetal fluoride, and an alkali earth metal fluoride; and a substancehaving an electron transporting property. The second mixed layers 3422 aand 3422 b each contain an aromatic hydrocarbon and a metal oxide.Further, the first mixed layer 3421 a is provided to be closer to thefirst electrode 3401 than the second mixed layer 3422 a, whereas thefirst mixed layer 3421 b is provided to be closer to the first electrode3401 than the second mixed layer 3422 b. A hole transporting layer 3412a is provided between the first electrode 3401 and the first lightemitting layer 3413 a, a hole transporting layer 3412 b is providedbetween the second mixed layer 3422 a and the second light emittinglayer 3413 b, and a hole transporting layer 3412 c is provide betweenthe second mixed layer 3422 b and the third light emitting layer 3413 c.Further, an electron transporting layer 3414 a is provided between thefirst light emitting layer 3413 a and the first mixed layer 3421 a, anelectron transporting layer 3414 b is provided between the second lightemitting layer 3413 b and the first mixed layer 3421 b, and an electrontransporting layer 3414 c is provided between the third light emittinglayer 3413 c and the second electrode 3402. Furthermore, a holeinjecting layer 3411 is provided between the first electrode 3401 andthe hole transporting layer 3412 a, and an electron injecting layer 3415is provided between the second electrode 3402 and the electrontransporting layer 3414 c. Each of the first light emitting layer 3413a, the second light emitting layer 3413 b, and the third light emittinglayer 3413 c contains a light emitting substance. When voltage isapplied to each of the first electrode 3401 and the second electrode3402 so that potential of the first electrode 3401 is higher than thatof the second electrode 3402, holes and electrons are recombined in eachof the light emitting layers, and the light emitting substance containedin each of the light emitting layers is excited by excitation energygenerated by the recombination. The excited light emitting substanceemits light upon returning to a ground state. Note that the lightemitting substances contained in each of the light emitting layers maybe the same or different from one another.

The first electrode 3401 is preferably formed using a substance having ahigh work function such as indium tin oxide, indium tin oxide containingsilicon oxide, indium oxide containing 2 to 20 wt % zinc oxide, gold(Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr),molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), ortantalum nitride. Note that, when a layer containing an aromatichydrocarbon and a metal oxide is provided as substitute for the holeinjecting layer 3411, the first electrode 3401 can be formed by using asubstance having a low work function such as aluminum and magnesium.

The second electrode 3402 is preferably formed using a substance havinga low work function such as aluminum and magnesium. When a layergenerating electrons is provided between the second electrode 3402 andthe third light emitting layer 3413 c, however, a substance having ahigh work function such as indium tin oxide, indium tin oxide containingsilicon oxide, indium oxide containing 2 to 20 wt % zinc oxide, gold(Au), platinum (Pt), nickel (Ni), tungsten (W), chromium (Cr),molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium (Pd), ortantalum nitride; and the like can also be used. Therefore, the secondelectrode 3402 may be formed by appropriately selecting either asubstance having a high work function or a substance having a low workfunction in accordance with a characteristic of a layer provided betweenthe second electrode 3402 and the third light emitting layer 3413 c.

In the light emitting element of this embodiment mode, the first mixedlayers 3421 a and 3421 b are much the same as the first mixed layer 3215described in Embodiment Mode 5. Further, the second mixed layers 3422 aand 3422 b are much the same as the second mixed layer 3216 described inEmbodiment Mode 5. Furthermore, the first light emitting layer 3413 a,the second light emitting layer 3413 b, and the third light emittinglayer 3413 c are much the same as the light emitting layer 3213described in Embodiment Mode 5. Moreover, the hole injecting layer 3411,the hole transporting layers 3412 a, 3412 b, and 3412 c, the electrontransporting layers 3414 a, 3414 b, and 3414 c, and the electroninjecting layer 3415 are also much the same as the respective layersdenoted by the same names in Embodiment Mode 4 or 5.

By forming the light emitting element having the second mixed layers3422 a and 3422 b each containing an aromatic hydrocarbon and a metaloxide as described above, defects due to crystallization of a layerbetween the pair of electrodes can be reduced. More specifically, ashort-circuiting which is caused as a result of generation of concavityand convexity due to the crystallization can be reduced as compared witha light emitting element having a layer only containing an aromatichydrocarbon or a metal oxide. Further, by providing the second mixedlayers 3422 a and 3422 b each containing an aromatic hydrocarbon and ametal oxide, a favorable light emitting element can be obtained wheredamage due to sputtering of a layer formed using an organic compoundsuch as the light emitting layer is not easily generated as comparedwith a light emitting element having a structure in which layers formedby sputtering like a layer made from indium tin oxide are providedbetween each light emitting layers.

Embodiment Mode 7

A light emitting element of the present invention can reduce anoperational defect due to crystallization of a layer provided between apair of electrodes. Further, by increasing the thickness of a mixedlayer containing an aromatic hydrocarbon and a metal oxide, which isprovided between a pair of electrodes, a short-circuiting between thepair of electrodes can be prevented. Furthermore, by changing thethickness of the mixed layer, a light path length can be adjusted orlight extraction efficiency to an external portion can be improved, andlight emission with fine color purity can be obtained. Therefore, byusing the light emitting element of the present invention as a pixel, afavorable light emitting device with less display defects due tooperational defects of the light emitting element can be obtained.Moreover, by using the light emitting element of the present inventionas a pixel, a light emitting device being capable of providing imageswith favorable colors can be obtained. By using the light emittingelement of the present invention as a light source, a light emittingdevice capable of illuminating favorably with less defects due to anoperational defect of the light emitting element can be obtained.

In this embodiment mode, a circuit structure of a light emitting devicehaving a display function and a method for driving thereof will bedescribed with reference to FIGS. 15 to 18.

FIG. 15 is a view showing a schematic block diagram seen from a topsurface of a light emitting device to which the present invention isapplied. In FIG. 15, a pixel portion 6511, a source signal line drivercircuit 6512, a wiring gate signal line driver circuit 6513, and anerasing gate signal line driver circuit 6514 are provided over asubstrate 6500. The source signal line driver circuit 6512, the writinggate signal line driver circuit 6513, and the erasing gate signal linedriver circuit 6514 are respectively connected to FPCs (flexible printedcircuits) 6503, which are external input terminals, through wiringgroups. The source signal line driver circuit 6512, the writing gatesignal line driver circuit 6513, and the erasing gate signal line drivercircuit 6514 receive video signals, clock signals, start signals, resetsignals and the like from the FPCs 6503, respectively. The FPCs 6503 areattached with printed wiring boards (PWBs) 6504. Further, a drivercircuit portion is not necessary to be formed over the same substrate asthe pixel portion 6511 as described above. For example, the drivercircuit portion may be provided outside of the substrate by utilizing aTCP in which an IC chip is mounted over an FPC having a wiring pattern,or the like.

A plurality of source signal lines extending in columns are aligned inrows in the pixel portion 6511. Also, power supply lines are aligned inrows. A plurality of gate signal lines extending in rows are aligned incolumns in the pixel portion 6511. In addition, a plurality of circuitseach including a light emitting element are aligned in the pixel portion6511.

FIG. 16 is a diagram showing a circuit for operating one pixel. Thecircuit as shown in FIG. 16 comprises a first transistor 901, a secondtransistor 902, and a light emitting element 903.

Each of the first and second transistors 901 and 902 is a three terminalelement including a gate electrode, a drain region, and a source region.A channel region is interposed between the drain region and the sourceregion. Since a region serving as the source region and a region servingas the drain region are changed depending on a structure of atransistor, an operational condition, and the like, it is difficult todetermine which region serves as the source region or the drain region.Therefore, regions serving as the source or the drain are denoted as afirst electrode of a transistor and a second electrode of a transistorin this embodiment mode, respectively.

A gate signal line 911 and a writing gate signal line driver circuit 913are provided to be electrically connected or disconnected to each otherby a switch 918. The gate signal line 911 and an erasing gate signalline driver circuit 914 are provided to be electrically connected ordisconnected to each other by a switch 919. A source signal line 912 isprovided to be electrically connected to either a source signal linedriver circuit 915 or a power source 916 by a switch 920. A gate of thefirst transistor 901 is electrically connected to the gate signal line911. The first electrode of the first transistor is electricallyconnected to the source signal line 912 while the second electrode ofthe first transistor is electrically connected to a gate electrode ofthe second transistor 902. The first electrode of the second transistor902 is electrically connected to a current supply line 917 while thesecond electrode of the second transistor is electrically connected toone electrode included in the light emitting element 903. Further, theswitch 918 may be included in the writing gate signal line drivercircuit 913. The switch 919 may also be included in the erasing gatesignal line driver circuit 914. In addition, the switch 920 may beincluded in the source signal line driver circuit 915.

The arrangement of transistors, light emitting elements and the like inthe pixel portion is not particularly limited. For example, thearrangement as shown in a top view of FIG. 17 can be employed. In FIG.17, a first electrode of a first transistor 1001 is connected to asource signal line 1004 while a second electrode of the first transistoris connected to a gate electrode of a second transistor 1002. A firstelectrode of the second transistor is connected to a current supply line1005 and a second electrode of the second transistor is connected to anelectrode 1006 of a light emitting element. A part of the gate signalline 1003 functions as a gate electrode of the first transistor 1001.

Next, the method for driving the light emitting device will be describedbelow. FIG. 18 is a diagram explaining an operation of one frame withtime. In FIG. 18, a horizontal direction indicates time passage while alongitudinal direction indicates the number of scanning stages of a gatesignal line.

When an image is displayed on the light emitting device of the presentinvention, a rewriting operation and a displaying operation are carriedout alternately during a displaying period. The number of the rewritingoperations is not particularly limited. However, the rewriting operationis preferably performed about 60 times a second so that a person whowatches a displayed image does not detect flicker in the image. A periodof operating the rewriting operation and the displaying operation of oneimage (one frame) is, herein, referred to as one frame period.

As shown in FIG. 18, one frame is divided into four sub-frames 501, 502,503 and 504 including writing periods 501 a, 502 a, 503 a and 504 a andholding periods 501 b, 502 b, 503 b and 504 b. The light emittingelement applied with a signal for emitting light, emits light during theholding periods. The length ratio of the holding periods in the firstsub-frame 501, the second sub-frame 502, the third sub-frame 503 and thefourth sub-frame 504 satisfies 2³:2²:2¹:2⁰=8:4:2:1. This allows thelight emitting device to exhibit 4-bit gray scale. Further, the numberof bits and the number of gray scales are not limited to those as shownin this embodiment mode. For instance, one frame may be divided intoeight sub-frames so as to achieve 8-bit gray scale.

The operation in one frame will be described. In the sub-frame 501, thewriting operation is first performed in a 1^(st) row to a last row,sequentially. Therefore, the starting time of the writing periods isvaried for each row. The holding period 501 b sequentially starts in therows in which the writing period 501 a has been terminated. In theholding period 501 b, a light emitting element applied with a signal foremitting light, remains in a light emitting state. Upon terminating theholding period 501 b, the sub-frame 501 is changed to the next sub-frame502 sequentially in the rows. In the sub-frame 502, a writing operationis sequentially performed in the 1^(st) row to the last row in the samemanner as the sub-frame 501. The above-mentioned operations are carriedout repeatedly up to the holding period 504 b of the sub-frame 504 andthen terminated. After terminating the operation in the sub-frame 504,an operation in the next frame starts. Accordingly, the sum of thelight-emitting time in respective sub-frames corresponds to the lightemitting time of each light emitting element in one frame. By changingthe light emitting time for each light emitting element and combiningsuch the light emitting elements variously within a pixel portion,various display colors with different brightness and differentchromaticity can be obtained.

When the holding period is intended to be forcibly terminated in the rowin which the writing period has already been terminated and the holdingperiod has started prior to terminating the writing operation up to thelast row as shown in the sub-frame 504, an erasing period 504 c ispreferably provided after the holding period 504 b so as to stop lightemission forcibly. The row where light emission is forcibly stopped,does not emit light for a certain period (this period is referred to asa non light emitting period 504 d). Upon terminating the writing periodin the last row, a writing period of a next sub-frame (or, a next frame)immediately starts from a first row, sequentially. This can prevent thewriting period in the sub-frame 504 from overlapping with the writingperiod in the next sub-frame.

Although the sub-frames 501 to 504 are arranged in order of descendingthe length of the holding period in this embodiment mode, they are notnecessary to be arranged in this order. For example, the sub-frames maybe arranged in ascending order of the length of the holding period.Alternatively, the sub-frames may be arranged in random order. Inaddition, these sub-frames may further be divided into a plurality offrames. That is, scanning of gate signal lines may be performed atseveral times during a period of supplying same video signals.

The operations in the wiring period and the erasing period of thecircuits as shown in FIG. 16 will be described below.

First, the operation in the writing period will be described. In thewriting period, the gate signal line 911 in the n-th row (n is a naturalnumber) is electrically connected to the writing gate signal line drivercircuit 913 via the switch 918. The gate signal line 911 in the n-th rowis electrically disconnected to the erasing gate signal line drivercircuit 914. The source signal line 912 is electrically connected to thesource signal line driver circuit 915 via the switch 920. In this case,a signal is input in a gate of the first transistor 901 connected to thegate signal line 911 in the n-th row (n is a natural number), therebyturning the first transistor 901 on. At this moment, video signals aresimultaneously input in the source signal lines in the first to lastcolumns. Further, the video signals input from the source signal line912 in each column are independent from one another. The video signalsinput from the source signal line 912 are input in a gate electrode ofthe second transistor 902 via the first transistor 901 connected to therespective source signal lines. It is decided whether the light emittingelement 903 emits light or emits no light depending on a signal input inthe second transistor 902. For instance, when the second transistor 902is of a P-channel type, the light emitting element 903 emits light byinputting a low level signal in the gate electrode of the secondtransistor 902. On the other hand, when the second transistor 902 is ofan N-channel type, the light emitting element 903 emits light byinputting a high level signal in the gate electrode of the secondtransistor 902.

Next, the operation in the erasing period will be described. In theerasing period, the gate signal line 911 in the n-th row (n is a naturalnumber) is electrically connected to the erasing gate signal line drivercircuit 914 via the switch 919. The gate signal line 911 in the n-th rowis electrically disconnected to the writing gate signal line derivercircuit 913. The source signal line 912 is electrically connected to thepower source 916 via the switch 920. In this case, upon inputting asignal in the gate of the first transistor 901, which is connected tothe gate signal line 911 in the n-th row, the first transistor 901 isturned on. At this time, erasing signals are simultaneously input in thefirst to last columns of the source signal lines. The erasing signalsinput from the source signal line 912 are input in the gate electrode ofthe second transistor 902 via the first transistor 901, which isconnected to each source signal line. At this time, the current supplyline 917 and the light emitting element 903 becomes an electricallynon-conductive state by a signal input in the second transistor 902.This makes the light emitting element 903 emit no light forcibly. Forexample, when the second transistor 902 is of a P-channel type, thelight emitting element 903 emits no light by inputting a high levelsignal in the gate electrode of the second transistor 902. On the otherhand, when the second transistor 902 is of an N-channel type, the lightemitting element 903 emits no light by inputting a low level signal inthe gate electrode of the second transistor 902.

Further, in the erasing period, a signal for erasing is input in then-th row (n is a natural number) by the above-mentioned operation.However, as mentioned above, the n-th row sometimes remains in theerasing period while another row (e.g., an m-th row (m is a naturalnumber)) remains in the writing period. In this case, since a signal forerasing is necessary to be input in the n-th row and a signal forwriting is necessary to be input in the m-th row by utilizing the sourcesignal line in the same column, the after-mentioned operation will bepreferably carried out.

After the light emitting element 903 in the n-th row becomes a non-lightemitting state by the above-described operation in the erasing period,the gate signal line 911 and the erasing gate signal line driver circuit914 are immediately disconnected to each other and the source signalline 912 is connected to the source signal line driver circuit 915 byturning the switch 920 on/off. The gate signal line 911 and the writinggate signal line driver circuit 913 are connected to each other whilethe source signal line and the source signal line driver circuit 915 areconnected to each other. A signal is selectively input in the signalline in the m-th row from the writing gate signal line driver circuit913 and the first transistor is turned on while signals for writing areinput in the source signal lines in the first to last columns from thesource signal line driver circuit 915. By inputting these signals, thelight emitting element in the m-th row emits light or no light.

After terminating the writing period in the m-th row as mentioned above,the erasing period immediately starts in the n+1-th row. Therefore, thegate signal line 911 and the writing gate signal line driver circuit 913are disconnected to each other while the source signal line is connectedto the power source 916 by turning the switch 920 on/off. Also, the gatesignal line 911 and the writing gate signal line driver circuit 913 aredisconnected to each other while the gate signal line 911 is connectedto the erasing gate signal line driver circuit 914. A signal isselectively input in the gate signal line in the n+1-th row from theerasing gate signal line driver circuit 914 to input a signal forturning on the first transistor in the first transistor while an erasingsignal is input therein from the power source 916. Upon terminating theerasing period in the n+1-th row in this manner, the writing periodimmediately starts in the m+1-th row. The erasing period and the writingperiod may be repeated alternately until the erasing period of the lastrow in the same manner.

Although the writing period in the m-th row is provided between theerasing period in the n-th row and the erasing period of the n+1-th rowin this embodiment mode, the present invention is not limited thereto.The writing period of the m-th row may be provided between the erasingperiod in the n−1-th row and the erasing period in the n-th row.

Furthermore, in this embodiment mode, when the non-light emitting period504 d is provided like the sub-frame 504, the operation of disconnectingthe erasing gate signal line driver circuit 914 from one gate signalline while connecting the writing gate signal line driver circuit 913 toother gate signal line, is carried out repeatedly. This operation may beperformed in a frame in which a non-light emitting period is notparticularly provided.

Embodiment Mode 8

One mode of a light emitting device including a light emitting elementof the present invention will be described with reference to crosssectional views of FIGS. 19A to 19C.

In each of FIGS. 19A to 19C, a transistor 11 that is provided fordriving a light emitting element 12 of the present invention issurrounded by a dashed line. The light emitting element 12 is a lightemitting element of the present invention as described in EmbodimentMode 1 through 6, which includes a layer 15 in which a light emittinglayer and a mixed layer containing an aromatic hydrocarbon and a metaloxide are laminated, between a first electrode 13 and a second electrode14. A drain of the transistor 11 and the first electrode 13 areelectrically connected to each other via a wiring 17 that passes througha first interlayer insulating film 16 (16 a, 16 b and 16 c). The lightemitting element 12 is isolated from other adjacent light emittingelement by a partition wall layer 18. A light emitting device havingsuch a structure is provided over a substrate 10 in this embodimentmode.

The transistor 11 shown in each of FIGS. 19A to 19C is of a top-gatetype in which a gate electrode is provided on a semiconductor layer at aside opposite to the substrate. Further, the structure of the transistor11 is not particularly limited thereto, and for example, a bottom-gatetype structure may be employed. In the case of the bottom-gate type,either a structure in which a protection film is formed over asemiconductor layer forming a channel (a channel protection type) or astructure in which a semiconductor layer forming a channel is partlyetched (a channel-etched type) may be used.

Furthermore, a semiconductor layer included in the transistor 11 may beformed using any one of a crystalline semiconductor, an amorphoussemiconductor, a semiamorphous semiconductor, and the like.

Specifically, the semiamorphous semiconductor has an intermediatestructure between an amorphous structure and a crystalline structure(including a single crystal structure and a polycrystalline structure),and a third condition that is stable in terms of free energy. Thesemiamorphous semiconductor further includes a crystalline region havinga short range order along with lattice distortion. A crystal grain witha size of 0.5 to 20 nm is included in at least a part of a semiamorphoussemiconductor film. Raman spectrum is shifted toward lower wave numbersthan 520 cm⁻¹. The diffraction peaks of (111) and (220), which arebelieved to be derived from Si crystal lattice, are observed in thesemiamorphous semiconductor by the X-ray diffraction. The semiamorphoussemiconductor contains hydrogen or halogen of at least 1 atom % or moreso as to terminate dangling bonds. The semiamorphous semiconductor isalso referred to as a so-called microcrystalline semiconductor. Thesemiamorphous semiconductor is formed by glow discharge decomposition(plasma CVD) with a gas such as SiH₄, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, orSiF₄. The gas containing silicon may also be diluted with H₂, or amixture of H₂ and one or more of rare gas elements of one or more of He,Ar, Kr, and Ne. The dilution ratio is set to be in the range of 1:2 to1:1,000. The pressure is set to be approximately in the range of 0.1 to133 Pa. The power supply frequency is set to be 1 to 120 MHz, andpreferably, 13 to 60 MHz. A substrate heating temperature may be set tobe 300° C. or less, and preferably, 100 to 250° C. With respect toimpurity elements contained in the film, each concentration ofimpurities for atmospheric constituents such as oxygen, nitrogen andcarbon, is preferably set to be 1×10²⁰ /cm³ or less. In particular, theoxygen concentration is set to be 5×10¹⁹ /cm³ or less, and preferably,1×10¹⁹ /cm³ or less.

As a specific example of a crystalline semiconductor layer, asemiconductor layer made from single crystal silicon, polycrystallinesilicon, silicon germanium, or the like can be given. The crystallinesemiconductor layer may be formed by laser crystallization. For example,the crystalline semiconductor layer may be formed by crystallizationwith use of a solid phase growth method using nickel or the like.

When a semiconductor layer is formed using an amorphous substance, e.g.,amorphous silicon, it is preferable to use a light emitting devicecomprising a circuit which includes only N-channel transistors as thetransistor 11 and other transistor (a transistor included in a circuitfor driving a light emitting element). Alternatively, a light emittingdevice comprising a circuit which includes either N-channel transistorsor P-channel transistors, may be employed. Also, a light emitting devicecomprising a circuit which includes both an N-channel transistor and aP-channel transistor, may be used.

The first interlayer insulating film 16 may include either plural layersas shown in FIGS. 19A to 19C or a single layer. Specifically, aninterlayer insulating film 16 a is formed using an inorganic materialsuch as silicon oxide and silicon nitride. An interlayer insulatinglayer 16 b is formed using acrylic, siloxane (which is a substance thathas a skeleton structure formed by a silicon (Si)-oxygen (O) bond andincludes at least hydrogen in a substituent), or a substance that can beformed by applying a liquid such as silicon oxide. An interlayerinsulating film 16 c is formed using a silicon nitride film containingargon (Ar). Further, the substances constituting the respective layersare not particularly limited thereto. Therefore, substances other thanthe above-mentioned substances may also be employed. Alternatively, alayer formed using a substance other than the above mentioned substancesmay also be provided in combination with the above described layers.Accordingly, the first interlayer insulating film 16 may be formed byusing both an inorganic material and an organic material or by usingeither an inorganic material or an organic material.

The edge portion of the partition wall layer 18 preferably has a shapein which the radius of curvature is continuously varied. This partitionwall layer 18 is formed by using acrylic, siloxane, resist, siliconoxide, or the like. Further, the partition wall layer 18 may be formedusing any one or both an inorganic film and an organic film.

Each of FIGS. 19A and 19C shows the structure in which only the firstinterlayer insulating film 16 is provided between the transistors 11 andthe light emitting elements 12. Alternatively, as shown in FIG. 19B, thefirst interlayer insulating film 16 (first interlayer insulating layers16 a and 16 b) and a second interlayer insulting film 19 (secondinterlayer insulting layers 19 a and 19 b) may be provided between thetransistor 11 and the light emitting element 12. In the light emittingdevice as shown in FIG. 19B, the first electrode 13 passes through thesecond interlayer insulating film 19 to be connected to the wiring 17.

The second interlayer insulating film 19 may include either plurallayers or a single layer as well as the first interlayer insulating film16. A second interlayer insulating film 19 a is formed using acrylic,siloxane, or a substance, which can be formed by applying a liquid suchas silicon oxide. A second interlayer insulating film 19 b is formedusing a silicon nitride film containing argon (Ar). The substancesconstituting the respective layers of the second interlayer insulatingfilm are not particularly limited thereto. Therefore, substances otherthan the above-mentioned substances may be employed. Alternatively, alayer made from a substance other than the above-mentioned substancesmay be provided in combination with the films 19 a and 19 b.Accordingly, the second interlayer insulating film 19 may be formed byusing both an inorganic material and an organic material or by usingeither an inorganic material or an inorganic material.

When the first electrode and the second electrode are both formed usinga substance with a light transmitting property in the light emittingelement 12, light generated in the light emitting element can be emittedthrough both the first electrode 13 and the second electrode 14 as shownin arrows in FIG. 19A. When only the second electrode 14 is formed usinga substance with a light transmitting property, light generated in thelight emitting element 12 can be emitted only through the secondelectrode 14 as shown in an arrow of FIG. 19B. In this case, the firstelectrode 13 is preferably formed using a material with highreflectance. Alternatively, a film (reflection film) formed using amaterial with high reflectance is preferably provided underneath thefirst electrode 13. When only the first electrode 13 is formed using asubstance with a light transmitting property, light generated in thelight emitting element 12 can be emitted only through the firstelectrode 13 as shown in an arrow of FIG. 19C. In this case, the secondelectrode 14 is preferably formed using a material with high reflectanceor a reflection film is preferably provided over the second electrode14.

Moreover, the layer 15 in which the layer containing a light emittingsubstance and a layer having low activation energy of electricalconductivity are laminated, may be stacked in the light emitting element12 so as to operate the light emitting element when applying voltagethereto so that a potential of a first electrode 13 is higher than thatof a second electrode 14. Alternatively, the layer 15 in which the layercontaining a light emitting substance and a layer having low activationenergy of electrical conductivity are laminated, may be stacked in thelight emitting element 12 so as to operate the light emitting elementwhen applying voltage to the light emitting element so that a potentialof a second electrode 14 is lower than that of a first electrode 13. Inthe former case, a transistor 11 is an N-channel transistor. In thelatter case, a transistor 11 is a P-channel transistor.

As set forth above, an active matrix light emitting device, whichcontrols a light emitting element by using a transistor, is described inthis embodiment mode. Alternatively, a passive light emitting device,which drives a light emitting element without providing a drivingelement such as a transistor, may also be employed. FIG. 20 shows aperspective view of a passive light emitting device manufactured inaccordance with the present invention. In FIG. 20, a layer 955 having amultilayer structure, which includes a layer containing an aromatichydrocarbon and a metal oxide, a light emitting layer, and the like, isprovided between an electrode 952 and an electrode 953. An edge portionof the electrode 952 is covered with an insulating layer 953. Apartition wall layer 954 is provided over the insulating layer 953.Sidewalls of the partition wall layer 954 are inclined so that a gapbetween one sidewall and the other sidewall is narrowed toward a surfaceof a substrate. That is, a cross section of the partition wall layer 954in a short side direction has a trapezoidal shape in which a lower side(a side facing the same direction as a surface of the insulating layer953 and being in contact with the insulating layer 953) is shorter thanan upper side (a side facing the same direction as the surface of theinsulating film 953 and being not contact with the insulating layer953). Accordingly, by providing the partition wall layer 954, defects ofthe light emitting element due to static electricity can be prevented.In the passive light emitting device, utilizing a light emitting elementof the present invention, which is driven at low driving voltage, makesit possible to drive the light emitting element at low powerconsumption.

Embodiment Mode 9

In a light emitting element including a layer containing an aromatichydrocarbon and a metal oxide, which is provided between a pair ofelectrodes, concavity and convexity formed by crystallization of a layerprovided between a pair of electrodes or operational defects caused byshort-circuiting between the pair of electrodes due to concavity andconvexity, which is formed on surfaces of the electrodes, can bereduced. Therefore, a light emitting device using such a light emittingelement as a pixel is favorably operated with less display defects.Accordingly, by applying such a light emitting device to a displayportion, an electronic appliance with less false recognition and thelike of a display image due to display defects can be obtained.Moreover, a light emitting device using a light emitting element of thepresent invention as a light source can favorably emit light with lessdefects, which are caused by an operational defect of the light emittingelement. Therefore, by using such a light emitting device of the presentinvention as a lighting portion such as a backlight, operational defectssuch as dark spots, which are locally formed due to defects of the lightemitting element, can be reduced, thereby displaying images preferably.Furthermore, since a light emitting element in which a distance betweena light emitting layer and each electrode is controlled by changing athickness of a layer containing an aromatic hydrocarbon and a metaloxide has less variations in driving voltage due to the thickness of thelayer containing an aromatic hydrocarbon and a metal oxide, andtherefore, a light emitting device, which is driven at low drivingvoltage and emits light with good color purity, can be obtained.Therefore, by using such a light emitting device for a display portion,an electronic appliance requiring less power and being capable ofproviding images with good colors can be obtained.

Examples of electronic appliances mounted with light emitting devices ofthe present invention will be described in FIGS. 21A to 21C.

FIG. 21A shows a personal computer manufactured according to the presentinvention, including a main body 5521, a housing 5522, a display portion5523, a keyboard 5524, and the like. By incorporating a light emittingdevice using a light emitting element of the present invention asdescribed in Embodiment Mode 1 or 2 (e.g., a light emitting deviceincluding the structure as described in Embodiment Mode 3 or 4) in adisplay portion, a personal computer having the display portion, whichcan provide display images with excellent colors along with less defectsand less false recognition of display images, can be completed. Further,when a light emitting device using a light emitting element of thepresent invention as a light source is incorporated as a backlight, apersonal computer can be completed. Specifically, as shown in FIG. 22, alighting device in which a liquid crystal device 5512 and a lightemitting device 5513 are built in a housing 5511 and a housing 5514respectively may be incorporated as a display portion. As shown in FIG.22, an external input terminal 5515 is mounted on the liquid crystaldevice 5512, whereas an external input terminal 5516 is mounted on thelight emitting device 5513.

FIG. 21B shows a telephone manufactured according to the presentinvention, including a main body 5552, a display portion 5551, an audiooutput portion 5554, an audio input portion 5555, operation switches5556 and 5557, an antenna 5553, and the like. By incorporating a lightemitting device including a light emitting element of the presentinvention as a display portion, a telephone having the display portion,which can provide display images with excellent colors along with lessdefects and less false recognition of display images, can be completed.

FIG. 21C shows a television receiver manufactured according to thepresent invention, including a display portion 5531, a housing 5532,speakers 5533, and the like. By incorporating a light emitting devicehaving a light emitting element of the present invention as a displayportion, a television receiver having the display portion, which canprovide display images with excellent colors along with less defects andless false recognition of display images, can be completed.

As set forth above, a light emitting device of the present invention isextremely suitable as being used as a display portion for various kindsof electronic appliances. Further, electronic appliances are not limitedto the appliances described in this embodiment mode, and may be apicture recorder, a navigation device, and the like.

Embodiment 1

A method for manufacturing a light emitting element having a layercontaining an aromatic hydrocarbon and a metal oxide between a pair ofelectrodes and operation characteristics thereof will be describedbelow. In this embodiment, two light emitting elements (a light emittingelement 1 and a light emitting element 2) having the same structure withthe exception that molar ratios between the aromatic hydrocarbon and themetal oxide were different from each other were manufactured. Note thatin this embodiment, t-BuDNA was used as the aromatic hydrocarbon andmolybdenum oxide was used as the metal oxide.

As shown in FIG. 23, a first electrode 301 was formed over a substrate300 using indium tin oxide containing silicon oxide to have a thicknessof 110 nm. The first electrode was formed by sputtering.

Next, a first layer 311 containing t-BuDNA and molybdenum oxide wasformed over the first electrode 301 by co-evaporation. Note that in thisembodiment, the first layer 311 was formed by using molybdenum trioxideamong molybdenum oxide as an evaporation material. The thickness of thefirst layer 311 was set to be 120 nm. Further, a weight ratio betweent-BuDNA and molybdenum oxide contained in the light emitting element 1was set to be 1:0.5=t-BuDNA:molybdenum oxide (a molar ratio was1:1.7=t-BuDNA:molybdenum oxide). A weight ratio between t-BuDNA andmolybdenum oxide contained in the light emitting element 2 was set to be1:0.75=t-BuDNA:molybdenum oxide (a molar ratio was1:2.5=t-BuDNA:molybdenum oxide). Note that, co-evaporation indicates anevaporation method by which materials are evaporated from a plurality ofevaporation sources provided in one processing chamber, and theevaporated materials are deposited over an object to form a layer inwhich a plurality of substances are mixed.

Next, a second layer 312 was formed over the first layer 311 using NPBby evaporation. The thickness of the second layer 312 was set to be 10nm. The second layer 312 served as a hole transporting layer when eachof the light emitting elements was driven.

Subsequently, a third layer 313 containing Alq₃ and coumarin 6 wasformed over the second layer 312 by co-evaporation. The thickness of thethird layer 313 was set to be 37.5 nm. Further, a weight ratio betweenAlq₃ and coumarin 6 was set to be 1:0.01=Alq₃:coumarin 6 (a molar ratiowas 1:0.013=Alq₃:coumarin 6). Accordingly, coumarin 6 was dispersed in alayer formed of Alq₃. When each of the light emitting elements wasdriven, the third layer 313 served as a light emitting layer.

Next, a fourth layer 314 was formed over the third layer 313 using Alq₃by evaporation. The thickness of the fourth layer 314 was set to be 37.5nm. When each of the light emitting elements was driven, the fourthlayer 314 served as an electron transporting layer.

Then, a fifth layer 315 was formed over the fourth layer 314 usinglithium fluoride by evaporation. The thickness of the fifth layer 315was set to be 1 nm. When each of the light emitting elements was driven,the fifth layer 315 served as an electron injecting layer.

Next, a second electrode 302 was formed using aluminum over the fifthlayer 315 by evaporation to have a thickness of 200 nm.

Measurement results of operation characteristics of the light emittingelements manufactured above are shown in FIGS. 24 to 26. The operationcharacteristics thereof were measured by applying voltage to each of thelight emitting elements so that potential of the first electrode 301 washigher than that of the second electrode 302. FIG. 24 is a graph showingvoltage-luminance characteristics of the light emitting elements,wherein a horizontal axis indicates voltage (V) and a vertical axisindicates luminance (cd/m²). FIG. 25 is a graph showing voltage-currentcharacteristics of the light emitting elements, wherein a horizontalaxis indicates voltage (V) and a vertical axis indicates current (mA).FIG. 26 is a graph showing luminance-current efficiency characteristicsof the light emitting elements, wherein a horizontal axis indicatesluminance (cd/m²) and a vertical axis indicates current efficiency(cd/A). In FIGS. 24 to 26, plots represented by black circles representcharacteristics of the light emitting element 1 whereas plotsrepresented by white circles represent characteristics of the lightemitting element 2.

Comparative Example

As a comparative example with respect to the light emitting elementsmanufactured in Embodiment 1, a light emitting element in which a layeronly including t-BuDNA is provided between a pair of electrodes will bedescribed. Note that, differing from the light emitting elements 1 and 2described in Embodiment 1, in the light emitting element of thecomparative example, the layer only including t-BuDNA is providedinstead of the first layer 311 containing t-BuDNA and molybdenum oxide;however, other structure of the light emitting element of thecomparative example is the same as that of the light emitting elements 1and 2 described in Embodiment 1. A method for manufacturing the lightemitting element of the comparative example is not described here. Whenthe light emitting element of the comparative example was driven,results of plots represented by white triangles can be obtained.

When comparing Embodiment 1 with the comparative example, it is knownthat a favorable light emitting element being driven at low drivingvoltage, in which by providing the layer containing an aromatichydrocarbon and a metal oxide between the pair of electrodes, lightemission start voltage (time of starting light emission at luminance of1 cd/m² is referred to as light emission start, and voltage applied inthis time is referred to as light emission start voltage) is low andapplied voltage required for making the light emitting element emitlight at predetermined luminance is also low, can be obtained. Further,it is known that by providing the layer containing an aromatichydrocarbon and a metal oxide between the pair of electrodes, afavorable light emitting element with high current efficiency whenmaking the light emitting element emit light at predetermined luminance,can be obtained.

Embodiment 2

Voltage-current characteristics of three samples 1 to 3 each having alayer containing an aromatic hydrocarbon and a metal oxide between apair of electrodes and a sample 4 having a layer only including anaromatic hydrocarbon between a pair of electrodes were measured. As aresult, it is known that carriers were more easily injected from theelectrodes and electric conductivity was higher in the layer containingan aromatic hydrocarbon and a metal oxide than the layer only includingan aromatic hydrocarbon.

Measurement results of voltage-current characteristics are shown in FIG.27. In FIG. 27, a horizontal axis represents voltage (V) whereas avertical axis represents current (mA). Further, in FIG. 27, plotsrepresented by black circles represent characteristics of the sample 1,plots represented by black squares represent characteristics of thesample 2, plots represented by white squares represent characteristicsof the sample 3, and plots represented by white triangles representcharacteristics of the sample 4.

Each of the samples 1 to 3 used in the measurement had a structure inwhich a mixed layer (200 nm in thickness) containing both an aromatichydrocarbon and a metal oxide was provided between an electrode (110 nmin thickness) made from indium tin oxide containing silicon oxide and anelectrode (200 nm in thickness) made from aluminum. The sample 4 had astructure in which a layer (200 nm in thickness) made from an aromatichydrocarbon (t-BuDNA was used as an aromatic hydrocarbon in thisembodiment) was provided between an electrode (110 nm in thickness) madefrom indium tin oxide containing silicon oxide and an electrode (200 nmin thickness) made from aluminum. Weight ratios between the aromatichydrocarbon and the metal oxide contained in each layer provided betweenthe pair of electrodes were differed in the samples 1 to 3. In thesample 1, the weight ratio between the aromatic hydrocarbon and themetal oxide was set to be 1:0.5=t-BuDNA:molybdenum oxide. In the sample2, the weight ratio between the aromatic hydrocarbon and the metal oxidewas set to be 2:0.75=t-BuDNA:molybdenum oxide. In the sample 3, theweight ratio between the aromatic hydrocarbon and the metal oxide wasset to be 1:1=t-BuDNA:molybdenum oxide. Note that in this embodiment,each mixed layer was formed by using t-BuDNA as an aromatic hydrocarbonand molybdenum trioxide as a metal oxide.

Embodiment 3

An absorption spectrum of a sample 5 in which a mixed layer containingan aromatic hydrocarbon and a metal oxide was formed over a glasssubstrate by co-evaporation and an absorption spectrum of a sample 6 inwhich a layer only including an aromatic hydrocarbon was formed over aglass substrate by evaporation were measured. As a result of themeasurement, it was known that the samples 5 and 6 showed absorptionderived from charge transfer in a spectrum of 600 to 1,200 nm. Further,in this embodiment, the mixed layer of the sample 5 was formed usingt-BuDNA as the aromatic hydrocarbon and molybdenum trioxide as the metaloxide so that a weight ratio between t-BuDNA and molybdenum trioxide wasset to be 4:2=t-BuDNA:molybdenum trioxide=and the thickness of the mixedlayer was set to be 100 nm. Further, the layer only including the samearomatic hydrocarbon as the sample 6 was formed by using t-BuDNA as thearomatic hydrocarbon to have a thickness of 100 nm. Measurement resultsof absorption spectrums of each sample are shown in FIG. 28. In FIG. 28,a horizontal axis indicates a wavelength (nm) whereas a vertical axisindicates absorbance (with no unit). Further, a heavy curved lineindicates absorption spectrums of the sample 5 whereas a thin curvedline indicates absorption spectrums of the sample 6. FIG. 29 is a graphshowing absorbance (with no unit) per 1 μm in thickness of the mixedlayer based on data of FIG. 28. In FIG. 29, a horizontal axis indicatesa wavelength (nm) whereas a vertical axis indicates absorbance (with nounit) per 1 μm in thickness. According to FIG. 29, it is known thatabsorbance per 1 μm in thickness in a wavelength region of 450 to 650 nmis 0.3 to 0.8. Further, FIG. 30 is a graph showing wavelength dependenceof transmittance, which is obtained based on measurement results of theabsorption spectrums. In FIG. 29, a horizontal axis represents awavelength (nm) whereas a vertical axis represents transmittance (%).According to FIG. 30, it is known that transmittance with respect tolight in the spectrum of 450 to 650 nm is 83 to 93% and differencebetween the two extreme values is 10% or less, that is, changes intransmittance, which are dependent on the wavelengths of transmittedlight, are extremely small.

Embodiment 4

A sample 7 in which a mixed layer containing both an aromatichydrocarbon and metal oxide was formed over a quartz substrate byco-evaporation and a sample 8 in which a layer only including anaromatic hydrocarbon was formed over a quartz substrate by evaporationwere measured by an electron spin resonance (ESR) technique at a roomtemperature. As a result, it was known that unpaired electrons existedin the mixed layer containing both an aromatic hydrocarbon and a metaloxide, that is, charge transfer complexes were generated therein. Notethat in this embodiment mode, the mixed layer of the sample 7 was formedwith a thickness of 200 nm by using t-BuDNA as the aromatic hydrocarbonand molybdenum trioxide as the metal oxide so that a weight ratiobetween t-BuDNA and molybdenum trioxide was set to be4:2=t-BuDNA:molybdenum trioxide. Further, the layer only including thesame aromatic hydrocarbon as the sample 8 was formed using t-BuDNA asthe aromatic hydrocarbon to have a thickness of 200 nm. Measurementresults of the respective samples are shown in FIG. 31 and FIG. 32. FIG.31 shows ESR spectrums of the sample 7 whereas FIG. 32 shows ESRspectrums of the sample 8. According to FIG. 31 and FIG. 32, it is knownthat ESR spectrum showing existence of unpaired electrons was detected.Further, according to the ESR measurement, it was known that, in thesample 7, a g-value was 2.0027, a line width (AH) was 0.58 mT, and aspin concentration was 1.7×10²⁰ spin/cm³. Note that the spinconcentration is a value obtained by a calculation in a case where thethickness of the mixed layer was set to be 200 nm.

Embodiment 5

After manufacturing samples 9 to 20 in each of which a mixed layer or alayer only including an aromatic hydrocarbon was formed over a glasssubstrate, absorption spectrums of the respective samples were measuredto obtain wavelength dependence of transmittance based on themeasurement results. In each of the samples 9, 10, 12, 13, 14, 16, 17,19, and 20, a mixed layer containing both an aromatic hydrocarbon and ametal oxide was formed over a glass substrate by co-evaporation to havea thickness of 100 nm. Each of the samples 11, 15, and 18 was a samplein which a layer only including an aromatic hydrocarbon was formed tohave a thickness of 10 nm over a glass substrate by evaporation. In eachof the samples 9 to 20, substances used as an aromatic hydrocarbon,substances used as a metal oxide, and mixture ratios of these substancesare shown in Table 1. Note that in this embodiment, molybdenum trioxide(MoO₃) was used among molybdenum oxide as a metal oxide.

TABLE 1 Mixture ratio (weight ratio) Aromatic Aromatic hydrocarbon:MetalSample hydrocarbon Metal oxide oxide  9 t-BuDNA molybdenum oxide 4:1 10t-BuDNA molybdenum oxide 4:4 11 DNA Non 4:0 12 DNA molybdenum oxide 4:113 DNA molybdenum oxide 4:2 14 DNA molybdenum oxide 4:4 15 DPPA Non 4:016 DPPA molybdenum oxide 4:2 17 DPPA molybdenum oxide 4:4 18 DPAnth Non4:0 19 DPAnth molybdenum oxide 4:2 20 DPAnth molybdenum oxide 4:4 Notethat each of t-BuDNA, DNA, DPPA, and DPAnth is an aromatic hydrocarbonhaving 26 to 60 carbon atoms including a structure as shown below.

Graphs showing wavelength dependence of transmittance are shown in FIGS.33 to 36. In each of FIGS. 33 to 36, a horizontal axis represents awavelength (nm) whereas a vertical axis represents transmittance (%).According to FIGS. 33 to 36, it is known that in a case of forming amixed layer by combining any one of aromatic hydrocarbons having 26 to60 carbon atoms with a metal oxide, transmittance with respect to lightin a spectrum of 450 to 650 nm is within a range of 83 to 98%, andtherefore, such transmittance is extremely favorable. Further, it isalso known that change in transmittance, which is dependent on thewavelength of transmitted light, is extremely small, and therefore,light with any wavelength can be favorably transmitted.

Embodiment 6

Voltage-current characteristics of a sample 21 having a mixed layercontaining both an aromatic hydrocarbon and a metal oxide between a pairof electrodes were measured. As a result, it was known that the mixedlayer containing both an aromatic hydrocarbon and a metal oxide showedfavorable conductivity.

FIG. 37 shows measurement results of the voltage-currentcharacteristics. In FIG. 37, a horizontal axis represents voltage (V)whereas a vertical axis represents current (mA).

In the sample 21 used in the measurement, an element having a structurein which a mixed layer (200 nm in thickness) containing both an aromatichydrocarbon and a metal oxide was interposed between an electrode (110nm in thickness) made from indium tin oxide containing silicon oxide andan electrode (200 nm in thickness) made from aluminum, was formed over aglass substrate. A weight ratio of the substances contained in the mixedlayer was set to be 1:0.5=DNA:molybdenum oxide. Note that DNA was usedas the aromatic hydrocarbon in this embodiment. Further, as a metaloxide, molybdenum trioxide (MoO₃) was used among molybdenum oxide.

Embodiment 7

A method for manufacturing a light emitting element having a layercontaining an aromatic hydrocarbon and a metal oxide between a pair ofelectrodes and operation characteristics thereof will be describedbelow. Note that in this embodiment, three light emitting elements(light emitting elements 3 to 5) having a similar structure with theexception that thicknesses of mixed layers are different from oneanother were manufactured. Further, t-BuDNA was used as the aromatichydrocarbon whereas molybdenum oxide was used as the metal oxide in thisembodiment.

Further, each of the light emitting elements of this embodiment issimilar to the light emitting element described in Embodiment 1 sinceeach of the light emitting elements had a structure in which a mixedlayer, a hole transporting layer, a light emitting layer, an electrontransporting layer, and an electron injecting layer are provided betweena pair of electrodes. Accordingly, the light emitting elements of thisembodiment will be described with reference to FIG. 23, which was usedin explanation of Embodiment 1.

As shown in FIG. 23, a first electrode 301 was formed over a substrate300 using indium tin oxide containing silicon oxide to have a thicknessof 110 nm. Note that the first electrode was formed by sputtering.

Next, a first layer 311 containing t-BuDNA and molybdenum oxide wasformed over the first electrode 301 by co-evaporation. Note that in thisembodiment, the first layer 311 was formed by using molybdenum trioxideamong molybdenum oxide as an evaporation material. The thicknesses ofthe first layers 311 of the light emitting elements were different fromone another. In the light emitting element 3, the first layer 311 wasformed to have a thickness of 20. In the light emitting element 4, thefirst layer 311 was formed to have a thickness of 50 nm. In the lightemitting element 5, the first layer 311 was formed to have a thicknessof 150 nm. Further, in the first layer 311 of each of the light emittingelements 3 to 5, a concentration of molybdenum oxide was set to be 10volume %. Preferably, prior to being evaporated, molybdenum oxide washeated at 450 to 550° C. under an atmosphere containing nitrogen gas orinert gas so as to remove moisture contained in an evaporation material.This can prevent reduction in degree of vacuum so that evaporation canbe performed in more stable state.

Next, a second layer 312 was formed over the first layer 311 using NPBby evaporation. The thickness of the second layer 312 was set to be 10nm. The second layer 312 served as a hole transporting layer when eachof the light emitting elements was driven.

Subsequently, a third layer 313 containing Alq₃ andN,N′-diphenylquinacridone (abbreviation: DPQd) was formed over thesecond layer 312 by co-evaporation. The thickness of the third layer 313was set to be 40 nm. Further, a weight ratio between Alq₃ and DPQd wasset to be 1:0.005=Alq₃:DPQd. Accordingly, DPQd was dispersed in a layermade from Alq₃. When each of the light emitting elements was driven, thethird layer 313 served as a light emitting layer.

Next, a fourth layer 314 was formed over the third layer 313 using Alq₃by evaporation. The thickness of the fourth layer 314 was set to be 30nm. When each of the light emitting elements was driven, the fourthlayer 314 served as an electron transporting layer.

Then, a fifth layer 315 was formed over the fourth layer 314 usinglithium fluoride by evaporation. The thickness of the fifth layer 315was set to be 1 nm. When each of the light emitting elements was driven,the fifth layer 315 served as an electron injecting layer.

Next, a second electrode 302 was formed using aluminum over the fifthlayer 315 by evaporation to have a thickness of 200 nm.

Measurement results of operation characteristics of the light emittingelements 3 to 5 manufactured above are shown in FIGS. 38 to 41. Theoperation characteristics thereof were measured by applying voltage toeach of the light emitting elements so that potential of the firstelectrode 301 was higher than that of the second electrode 302. FIG. 38is a graph showing voltage-luminance characteristics of the lightemitting elements, wherein a horizontal axis indicates voltage (V) and avertical axis indicates luminance (cd/m²). FIG. 39 is a graph showingvoltage-current characteristics of the light emitting elements, whereina horizontal axis indicates voltage (V) and a vertical axis indicatescurrent (mA). FIG. 40 is a graph showing luminance-current efficiencycharacteristics of the light emitting elements, wherein a horizontalaxis indicates luminance (cd/m²) and a vertical axis indicates currentefficiency (cd/A). FIG. 41 is a graph showing luminance-electric powerefficiency characteristics of the light emitting elements, wherein ahorizontal axis indicates luminance (cd/m²) and a vertical axisindicates electric power efficiency (lm/W).

According to the above described results, it is known that both of thevoltage-luminance characteristics and the voltage-currentcharacteristics of the light emitting elements 3 to 5 were equal to eachother, and driving voltage of each the light emitting elements accordingto the present invention is not increased with increasing the thicknessof the mixed layer.

Embodiment 8

A method for manufacturing a light emitting element having a layercontaining an aromatic hydrocarbon and a metal oxide between a pair ofelectrodes and operation characteristics thereof will be describedbelow. Note that in this embodiment, two light emitting elements (lightemitting elements 6 and 7) having a similar structure with the exceptionthat substances used as the aromatic hydrocarbon were different fromeach other were manufactured. Further, DPPA or t-BuDBA was used as thearomatic hydrocarbon whereas molybdenum oxide was used as the metaloxide in this embodiment.

Further, each of the light emitting elements of this embodiment issimilar to the light emitting element described in Embodiment 1 sinceeach of the light emitting elements has a structure in which a mixedlayer, a hole transporting layer, a light emitting layer, an electrontransporting layer, and an electron injecting layer are provided betweena pair of electrodes. Accordingly, the light emitting elements of thisembodiment will be described with reference to FIG. 23, which was usedin explanation of Embodiment 1.

As shown in FIG. 23, a first electrode 301 was formed over a substrate300 using indium tin oxide containing silicon oxide to have a thicknessof 110 nm. Note that the first electrode was formed by sputtering.

Next, a first layer 311 containing an aromatic hydrocarbon andmolybdenum oxide was formed over the first electrode 301 byco-evaporation. In the sample 6, DPPA was used as the aromatichydrocarbon. In the sample 7, t-BuDBA was used as the aromatichydrocarbon. Note that t-BuDBA is a substance represented by astructural formula as follows. Note that in this embodiment, the firstlayer 311 was formed by using molybdenum trioxide among molybdenum oxideas an evaporation material. The thickness of the first layer 311 of eachof the samples was set to be 50 nm. Further, in the first layer 311 ofeach of the light emitting elements 6 and 7, a concentration ofmolybdenum oxide contained in the mixed layer was set to be 10 volume %.

Next, a second layer 312 was formed over the first layer 311 using NPBby evaporation. The thickness of the second layer 312 was set to be 10nm. The second layer 312 served as a hole transporting layer when eachof the light emitting elements was driven.

Subsequently, a third layer 313 containing Alq₃ andN,N′-diphenylquinacridone (abbreviation: DPQd) was formed over thesecond layer 312 by co-evaporation. The thickness of the third layer 313was set to be 40 nm. Further, a weight ratio between Alq₃ and DPQd wasset to be 1:0.005=Alq₃: DPQd. Accordingly, DPQd was dispersed in a layermade from Alq₃. When each of the light emitting elements was driven, thethird layer 313 served as a light emitting layer.

Next, a fourth layer 314 was formed using Alq₃ over the third layer 313by evaporation. The thickness of the fourth layer 314 was set to be 30nm. When each of the light emitting elements was driven, the fourthlayer 314 served as an electron transporting layer.

Then, a fifth layer 315 was formed using lithium fluoride over thefourth layer 314 by evaporation. The thickness of the fifth layer 315was set to be 1 nm. When each of the light emitting elements was driven,the fifth layer 315 served as an electron injecting layer.

Next, a second electrode 302 was formed using aluminum over the fifthlayer 315 by evaporation to have a thickness of 200 nm.

Measurement results of operation characteristics of the light emittingelements 6 and 7 manufactured above are shown in FIG. 42. The operationcharacteristics thereof were measured by applying voltage to each of thelight emitting elements so that potential of the first electrode 301 washigher than that of the second electrode 302. FIG. 42 is a graph showingluminance-electric power efficiency characteristics of the lightemitting elements, wherein a horizontal axis indicates luminance (cd/m²)and a vertical axis indicates electric power efficiency (lm/W). Notethat characteristics of the light emitting element 4 is also shown inFIG. 42.

According to FIG. 42, it is known that the light emitting elementaccording to the present invention operates favorably in a case of usingany combination of an aromatic hydrocarbon and a metal oxide.

Embodiment 9

Changes in luminance with accumulation of light emitting time andchanges in driving voltage with accumulation of light emitting time ofthe light emitting elements 4, 6, and 7 were measured. The measurementwas performed as follows. The manufactured light emitting elements weremoved in a glove box under a nitrogen atmosphere, and the light emittingelements are sealed by using a seal material in the glove box.Thereafter, current density required for making the light emittingelements emit light at luminance of 3,000 cd/m² in an initial state wasmeasured first. Then, current with current density required for makingthe light emitting elements emit light at luminance of 3,000 cd/m² in aninitial state was fed for a given time to make the light emittingelements emit light continuously. Thus, light emission luminance andapplied voltage for elapsed time were plotted. Note that in thisembodiment, current density required for making the light emittingelements emit light at a luminance of 3,000 cd/m² was 24 mA/cm² for eachof the light emitting elements 4, 6, and 7. Further, the measurement wasperformed under a room temperature (about 25° C.).

Measurement results are shown in FIGS. 43A and 43B. FIG. 43A is a graphshowing changes in luminance with accumulation of light emitting time,wherein a horizontal axis indicates light emitting time (hour) and avertical axis indicates luminance (a value relative to initial luminancewhen setting the initial luminance to 100). Further, FIG. 43B is a graphshowing changes in driving voltage with accumulation of light emittingtime, wherein a horizontal axis indicate light emitting time (hour) anda vertical axis indicates voltage (V) applied to the light emittingelements so as to feed current with current density required for makingthe light emitting elements emit light at luminance of 3,000 cd/m².According to FIG. 43A, it is known that each of the light emittingelements 4, 6, and 7 had less reduction in luminance with accumulationof the light emitting time and favorable life. Moreover, according toFIG. 43B, it is known that each of the light emitting elements 4, 6, and7 had less increase in voltage with accumulation of the light emittingtime, i.e., less increase in resistance with accumulation of the lightemitting time.

Embodiment 10

In this embodiment, a light emitting element of the present inventionhaving a layer containing an aromatic hydrocarbon and a metal oxidebetween a pair of electrodes (a light emitting element 8) and a lightemitting element of the comparative example having a layer onlycontaining an aromatic hydrocarbon (a light emitting element 9) weremanufactured. Measurement results of changes in luminance and voltagewith accumulation of light emitting time of each of the light emittingelements will be described.

First, a method for manufacturing the light emitting element 8 will bedescribed. Further, the light emitting element 8 is similar to the lightemitting element shown in FIG. 23 since five layers are provided betweena pair of electrodes in the light emitting element 8. Accordingly, thelight emitting element 8 will also be described with reference to FIG.23.

As shown in FIG. 23, a first electrode 301 was formed using indium tinoxide containing silicon oxide over a substrate 300 to have a thicknessof 110 nm. Note that the first electrode was formed by sputtering.

Next, a first layer 311 containing t-BuDNA and molybdenum oxide wasformed over the first electrode 301 by co-evaporation. Note that in thisembodiment, the first layer 311 was formed by using molybdenum trioxideamong molybdenum oxide as an evaporation material. The thickness of thefirst layer 311 was set to be 120 nm. Further, a weight ratio betweent-BuDNA and molybdenum oxide was set to be 1:0.5=t-BuDNA:molybdenumoxide (t-BuDNA:molybdenum oxide=1:1.7 in a molar ratio).

Next, a second layer 312 was formed using NPB over the first layer 311by evaporation. The thickness of the second layer 312 was set to be 10nm. The second layer 312 served as a hole transporting layer when thelight emitting element was driven.

Subsequently, a third layer 313 containing Alq₃ and coumarin 6 wasformed over the second layer 312 by co-evaporation. The thickness of thethird layer 313 was set to be 37.5 nm. Further, a weight ratio betweenAlq₃ and coumarin 6 was set to be 1:0.01=Alq₃:coumarin 6. Accordingly,coumarin 6 was dispersed in a layer made from Alq₃. When the lightemitting element was driven, the third layer 313 served as a lightemitting layer.

Next, a fourth layer 314 was formed using Alq₃ over the third layer 313by evaporation. The thickness of the fourth layer 314 was set to be 37.5nm. When the light emitting element was driven, the fourth layer 314served as an electron transporting layer.

Then, a fifth layer 315 was formed over the fourth layer 314 usinglithium fluoride by evaporation. The thickness of the fifth layer 315was set to be 1 nm. When the light emitting element was driven, thefifth layer 315 served as an electron injecting layer.

Next, a second electrode 302 was formed using aluminum over the fifthlayer 315 by evaporation to have a thickness of 200 nm.

The method for manufacturing the light emitting element 8 was describedabove. A structure of the light emitting element 9 is similar to that ofthe light emitting element 8 with the exception of a structure of thefirst layer 311. Accordingly, only a method for forming the first layer311 of the light emitting element 9 will be described below, and otherportions are based on the explanations related to the light emittingelement 8.

The first layer 311 of the light emitting element 9 was formed onlyusing t-BuDNA. That is, the first layer 311 did not contain a metaloxide. The thickness of the first layer 311 was set to be 50 nm.

After manufacturing the light emitting elements 8 and 9 as describedabove, the light emitting elements were sealed using a seal materialinside a sealing apparatus under a nitrogen atmosphere. Then, changes inluminance with accumulation of light emitting time and changes indriving voltage with accumulation of light emitting time of the lightemitting elements 8 and 9 were measured. The measurement was performedas follows. First, current density required for making the lightemitting elements emit light at luminance of 3,000 cd/m² in an initialstate was measured. Then, current with current density required formaking the light emitting elements emit light at luminance of 3,000cd/m² in an initial state was fed for a given time to make the lightemitting elements emit light continuously. Thus, light emissionluminance and applied voltage for elapsed time were plotted. Note thatin this embodiment, current density required for making the lightemitting element 8 emit light at luminance of 3,000 cd/m² was 23.05mA/cm². In this embodiment, current density required for making thelight emitting element 9 emit light at luminance of 3,000 cd/m² was24.75 mA/cm². Further, the measurement was performed under a roomtemperature (about 25° C.).

Measurement results are shown in FIGS. 44A and 44B. FIG. 44A is a graphshowing changes in luminance with accumulation of light emitting time,wherein a horizontal axis indicates light emitting time (hour) and avertical axis indicates luminance (a value relative to initial luminancewhen setting the initial luminance to 100). Further, FIG. 44B is a graphshowing changes in driving voltage with accumulation of light emittingtime, wherein a horizontal axis indicate light emitting time (hour) anda vertical axis indicates voltage (V) applied to the light emittingelements so as to feed current with current density required for makingthe light emitting elements emit light with luminance of 3,000 cd/m².

According to FIG. 44A, it is known that the light emitting element 8 hasextremely lesser reduction in luminance with accumulation of the lightemitting time and more favorable life than the light emitting element 9of the comparative example. Moreover, according to FIG. 44B, it is knownthat the light emitting element 8 has extremely lesser increase involtage with accumulation of the light emitting time, i.e., lesserincrease in resistance with accumulation of the light emitting time thanthe light emitting element 9 of the comparative example.

Embodiment 11

Examples of light emitting elements using DPAnth as an aromatichydrocarbon according to the present invention will be described. Inthis embodiment, three light emitting elements (light emitting elements10 to 12) having different mixture ratios between a metal oxide used incombination with DPAnth and DPAnth were manufactured. Note that a methodfor manufacturing the light emitting elements 10 to 12 were similar toone another with the exception that the mixture ratios between the metaloxide and DPAnth were changed from one another.

First, a method for manufacturing the light emitting elements 10 to 12will be described. Further, each of the light emitting elements issimilar to the light emitting element shown in FIG. 23 since five layersare provided between a pair of electrodes in each of the light emittingelements. Accordingly, the light emitting elements will be describedwith reference to FIG. 23.

As shown in FIG. 23, a first electrode 301 was formed using indium tinoxide containing silicon oxide over a substrate 300 to have a thicknessof 110 nm. Note that the first electrode was formed by sputtering.

Next, a first layer 311 containing an aromatic hydrocarbon andmolybdenum oxide was formed over the first electrode 301 byco-evaporation. DPAnt was used as the aromatic hydrocarbon. Note that inthis embodiment, the first layer 311 was formed by using molybdenumtrioxide among molybdenum oxide as an evaporation material. Thethickness of the first layer 311 of each of the light emitting elementswas set to be 50 nm. In the light emitting element 10, a volume ratio ofmolybdenum oxide contained in the first layer 311 was set to be 4 volume%. In the light emitting element 11, a volume ratio of molybdenum oxidecontained in the first layer 311 was set to be 7 volume %. In the lightemitting element 12, a volume ratio of molybdenum oxide contained in thefirst layer 311 was set to be 10 volume %.

Next, a second layer 312 was formed using NPB over the first layer 311by evaporation. The thickness of the second layer 312 was set to be 10nm. The second layer 312 served as a hole transporting layer when eachof the light emitting elements was driven.

Subsequently, a third layer 313 containing Alq₃ andN,N′-diphenylquinacridone (abbreviation: DPQd) was formed over thesecond layer 312 by co-evaporation. The thickness of the third layer 313was set to be 40 nm. Further, a weight ratio between Alq₃ and DPQd wasset to be 1:0.005=Alq₃:DPQd. Accordingly, DPQd was dispersed in a layermade from Alq₃. When each of the light emitting elements was driven, thethird layer 313 served as a light emitting layer.

Next, a fourth layer 314 was formed using Alq₃ over the third layer 313by evaporation. The thickness of the fourth layer 314 was set to be 30nm. When each of the light emitting elements was driven, the fourthlayer 314 served as an electron transporting layer.

Then, a fifth layer 315 was formed over the fourth layer 314 usinglithium fluoride by evaporation. The thickness of the fifth layer 315was set to be 1 nm. When each of the light emitting elements was driven,the fifth layer 315 served as an electron injecting layer.

Next, a second electrode 302 was formed using aluminum over the fifthlayer 315 by evaporation to have a thickness of 200 nm.

Measurement results of operation characteristics of the light emittingelements 10 to 12 manufactured above are shown in FIG. 45. The operationcharacteristics thereof were measured by applying voltage to each of thelight emitting elements so that potential of the first electrode 301 washigher than that of the second electrode 302. FIG. 45 is a graph showingvoltage-luminance characteristics of the light emitting elements,wherein a horizontal axis indicates voltage (V) and a vertical axisindicates luminance (cd/m²). Further, plots represented by whitetriangles represent characteristics of the light emitting element 10,plots represented by white circles represent characteristics of thelight emitting element 11, and plots represented by black diamondsrepresent characteristics of the light emitting element 12. According toFIG. 45, it is known that there are almost no changes involtage-luminance characteristics, which are dependent on the rate ofmolybdenum oxide contained in each of the first layers 331, and each ofthe light emitting elements 10 to 12 containing molybdenum oxide with 4to 10 volume % is operated favorably.

Embodiment 12

Examples of light emitting elements using DNA as an aromatic hydrocarbonaccording to the present invention will be described. In thisembodiment, three light emitting elements (light emitting elements 13 to15) having different mixture ratios between a metal oxide used incombination with DNA and DNA were manufactured. Note that a method formanufacturing the light emitting elements 13 to 15 were similar to oneanother with the exception that the mixture ratios between the metaloxide and DNA were changed from one another.

First, a method for manufacturing the light emitting elements 13 to 15will be described. Further, each of the light emitting elements issimilar to the light emitting element shown in FIG. 23 since five layersare provided between a pair of electrodes in each of the light emittingelements. Accordingly, the light emitting elements will be describedwith reference to FIG. 23.

As shown in FIG. 23, a first electrode 301 was formed using indium tinoxide containing silicon oxide over a substrate 300 to have a thicknessof 110 nm. The first electrode was formed by sputtering.

Next, a first layer 311 containing an aromatic hydrocarbon andmolybdenum oxide was formed over the first electrode 301 byco-evaporation. DNA was used as the aromatic hydrocarbon. Note that inthis embodiment, the first layer 311 was formed by using molybdenumtrioxide among molybdenum oxide as an evaporation material. Thethickness of the first layer 311 of each of the light emitting elementswas set to be 50 nm. In the light emitting element 13, a volume ratio ofmolybdenum oxide contained in the first layer 311 was set to be 4 volume%. In the light emitting element 14, a volume ratio of molybdenum oxidecontained in the first layer 311 was set to be 7 volume %. In the lightemitting element 15, a volume ratio of molybdenum oxide contained in thefirst layer 311 was set to be 10 volume %.

Next, a second layer 312 was formed using NPB over the first layer 311by evaporation. The thickness of the second layer 312 was set to be 10nm. The second layer 312 served as a hole transporting layer when eachof the light emitting elements was driven.

Subsequently, a third layer 313 containing Alq₃ andN,N′-diphenylquinacridone (abbreviation: DPQd) was formed over thesecond layer 312 by co-evaporation. The thickness of the third layer 313was set to be 40 nm. Further, a weight ratio between Alq₃ and DPQd wasset to be 1:0.005=Alq₃:DPQd. Accordingly, DPQd was dispersed in a layermade from Alq₃. When each of the light emitting elements was driven, thethird layer 313 served as a light emitting layer.

Next, a fourth layer 314 was formed using Alq₃ over the third layer 313by evaporation. The thickness of the fourth layer 314 was set to be 30nm. When each of the light emitting elements was driven, the fourthlayer 314 served as an electron transporting layer.

Then, a fifth layer 315 was formed using lithium fluoride over thefourth layer 314 by evaporation. The thickness of the fifth layer 315was set to be 1 nm. When each of the light emitting elements was driven,the fifth layer 315 served as an electron injecting layer.

Next, a second electrode 302 was formed using aluminum over the fifthlayer 315 by evaporation to have a thickness of 200 nm.

Measurement results of operation characteristics of the light emittingelements 13 to 15 manufactured above are shown in FIG. 46. The operationcharacteristics thereof were measured by applying voltage to each of thelight emitting elements so that potential of the first electrode 301 washigher than that of the second electrode 302. FIG. 46 is a graph showingvoltage-luminance characteristics of the light emitting elements,wherein a horizontal axis indicates voltage (V) and a vertical axisindicates luminance (cd/m²). Further, plots represented by whitetriangles represent characteristics of the light emitting element 13,plots represented by white circles represent characteristics of thelight emitting element 14, and plots represented by black diamondsrepresent characteristics of the light emitting element 15. According toFIG. 46, it is known that there are almost no changes involtage-luminance characteristics, which are dependent on the rate ofmolybdenum oxide contained in each of the first layers 331, and each ofthe light emitting elements 13 to 15 containing molybdenum oxide with 4to 10 volume % is operated favorably.

Note that in the embodiments of this specification, a rate of molybdenumoxide contained in a mixed layer is represented by a volume ratio insome cases. For example, when molybdenum oxide is contained by a volumeratio of 10 volume % and the volume ratio is converted into a massratio, the mass ratio between an organic compound and molybdenum oxidebecomes 4:1=the organic compound:molybdenum oxide).

This application is based on Japanese Patent Application serial No.2005-181806 filed in Japan Patent Office on Jun. 22, 2005 and JapanesePatent Application serial No. 2005-213708 filed in Japan Patent Officeon Jul. 25, 2005, the entire contents of which are hereby incorporatedby reference.

What is claimed is:
 1. A light emitting device comprising: a firstelectrode; a second electrode; a light emitting layer between the firstelectrode and the second electrode; and a mixed layer between the firstelectrode and the light emitting layer, wherein the mixed layercomprises a metal oxide and a compound comprising an anthraceneskeleton, wherein the metal oxide shows an electron accepting propertywith respect to the compound comprising an anthracene skeleton, andwherein a transmittance of the mixed layer with respect to light in aspectrum of 450 to 650 nm is 80% or more.
 2. A light emitting deviceaccording to claim 1, wherein a hole mobility of the compound comprisingan anthracene skeleton is greater than or equal to 1×10⁻⁶ cm²/Vs.
 3. Alight emitting device according to claim 1, wherein the compoundcomprising an anthracene skeleton has 14 to 60 carbon atoms.
 4. A lightemitting device according to claim 1, wherein the compound comprising ananthracene skeleton is any one of2-tert-butyl-9,10-di(2-naphthyl)anthracene;9,10-di(naphthalen-1-yl)-2-tert-butyl anthracene; anthracene;9,10-diphenylanthracene; 9,10-bis(3,5-diphenylphenyl)anthracene;9,10-di(naphthalen -2-yl)anthracene; 2-tert-butylanthracene;9,10-di(4-methylnaphthalen-1-yl)anthracene;9,10-bis[2-(naphthalen-1-yOphenyl]anthracene;2,3,6,7-tetramethyl-9,10-di(naphthalen -1-yl)anthracene;2,3,6,7-tetramethyl-9,10-di(naphthalen-2-yl)anthracene; bianthryl;10,10′-di(2-phenylphenyl)-9,9′-bianthryl; 10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl; and9,10-di(4-phenylphenyl)-2-tert-butylanthracene.
 5. A light emittingdevice according to claim 1, wherein the first electrode is an anode. 6.An electronic appliance comprising the light emitting device accordingto claim
 1. 7. A lighting device comprising the light emitting deviceaccording to claim
 1. 8. A light emitting device according to claim 1,wherein the compound comprising an anthracene skeleton does not have anarylamine skeleton.